Thunderbird:Testing:Antivirus Related Performance Issues


This article is not just about performance issues. Antivirus software can also cause odd behavior (bad, incorrect or unusual), and crashes (random or consistent, infrequent or frequent).

Much of the information below is from users like you.

If you have a problem that is not listed here, please add it to the list so other users can benifit. (create a wiki account, and click "edit")

Related Articles



Possible behaviors:

  • Mild to severe slowness
  • Not able to get new mail
  • OS hangs or crashes
  • Images not displaying

... while doing the following:

  • Starting Thunderbird
  • Opening file attachments
  • Selecting a different folder
  • Opening / displaying a message
  • Moving messages to a different folder
  • Replying to a message / Sending a message
  • Downloading or attempting to get new messages (receiving mail)
  • Initial mailbox indexing for global search

My Problem Cannot be Caused by Antivirus

My Antivirus software has been working great. Thunderbird is the only program that is affected. I even tried disabling my antivirus software.

Warning signWarning: Your antivirus software probably worked great for months or years, across many previous versions of Thunderbird. But this is NO guarantee it is working correctly now. Antivirus and Thunderbird have a complex relationship, and that relationship can be broken by updates or version changes of either software. Yes even if Thunderbird is the only software affected, even if your AV version did not change. AV vendors don't always play nice, and don't always adapt their software to new versions of other software on your computer, whether you know it or not they do make accommodations under the hood for how they interact with other software. In short, if TB performance is good when AV software has been disabled, doesn't make this fundamentally a bug in Thunderbird, although there are certainly areas of Thunderbird performance that can be improved.

First Steps

A quick test of the impact of antivirus software (AV) is starting Windows in safe mode, or starting Mac in safe mode. If your problem is gone with the operating system in safe mode, then the cause can be antivirus or some other software loading during the OS startup.

  • *NOTE, testing by disabling antivirus using AV settings has proven over the years to not be a perfect or sufficient test, because sometimes AV setting changes are not effective. Therefore the only perfect test is OS in safe mode.
  • If you determine AV to be a problem, then you must either disable the AV feature affecting Thunderbird (typically by excluding the Thunderbird profile), or change to a different AV program.
    • If you have altered Thunderbird to put your profile account data in a nonstandard file location then you MUST set your antivirus software to exclude these nonstandard locations.

Testing Suggests My Problem is Caused by Antivirus Software

If safe mode or other adjustments suggest that antivirus may be causing your problem, you have the following choices:

  • Adjust the exclusions in the antivirus software settings to stop the problem. %%
  • Turn off features in antivirus software that are not absolutely necessary. %% Some features are included to make the vendor look good against their compeition, but don't actually make you much safer.
  • Inform the antivirus vendor, and be persistent in getting past a canned response that their software is not at fault.
  • Find a different antivirus software or vendor that offers more reliable service.

%% You might find suggestions below, on an antivirus website or support site, or in a bug report which flags antivirus in the whiteboard.

Problem Antivirus and Anti-Malware Packages

Please see the sections above before proceeding.

Problems have been reported with:

Also, several products have caused pop download errors or slowness if incoming mail is scanned [45]

Unaffected Antivrus Packages

The following programs have generally been reported to work well with Thunderbird. (This is not proof that these products can't have problems)

  • AVG Antivirus - (revised) from [46] "[AVG] are no longer going to support the Thunderbird plugin because of the frequent updates of Thunderbird." And you may see problems with older versions (like 9.0.830). AVG may cause slow downloading of pop mail [47]. and AVG 9.0 may slow down sending with TB 3.1.x if set to scan outgoing messages) [48].
  • ANTIVIR 9 seems to work with Thunderbird 3 - from klint: "ANTIVIR 9 (Free version) runs fine, but ... there is no specific MAIL protection activated in the free version I have. I'm using the global protection to protect me from anything that could be launched from a mail."
  • (old assessment and perhaps no longer true ... ESET NOD32 version 8 appears to cause no issues. Although the security suite is another matter.)
  • (Others wanted, please add)

Configuration Notes and Solutions for specific Anti Virus products


Feb 2019: AVG preventing Thunderbird from retrieving emails. The status bar says connected and getting mail but no mail arrives or mail arrives and is marked as read. AVG topic support here

AVG can not scan SSL connections and requires you to include it's certificates into the Thunderbird certificate store. AVG reference here

If you use AVG secure VPN you need to set up your mail client. See here It is important to not that specifying an IP address removes flexibility from your mail connection and may cause issues if the provider changes the Internet Protocol (IP) address used for the server. Normally Domain Name Services shield us from such changes by looking up the IP address on the fly.

AVG causes intermittent Not Responding in Windows 10.[49]

AVG causes unable to connect to server/gmail [50]


McAfee has had major problems with Thunderbird over the years, causing crashes and poor performance, "Not Responding", "Unresponsive Script", and other issues. (For example 2023 Thunderbird gets stuck and I need to reboot SOLVED caused by McAfee[51], [52], [53], [54], [55], [56], [57], (in 2013), [58], [59], [60], [61], [62], [63], (perhaps), [64], [65], [66], [67], [68], [69], [70], [71], bug 592303, bug 1858380.)

We recommend you do all of the following:

  • Disable/delete McAfee Anti-Spam Thunderbird Extension in Thunderbird | Tools | Add-ons | Extensions (It is safe to remove the add-on, and is absolutely not needed to have excellent virus protection.) [72],[73]
  • Disable email attachment scanning in McAfee (it individually scans all emails which slows down Thunderbird) - Uncheck "Email attachments" under Real Time Scanning > Schedule and run scans > Settings > Scan these attachments and locations or (in older versions) Real Time Scanning > Settings > Scan these attachments and locations
  • Disable scanning of Thunderbird files and folders in McAfee (by default McAfee is NOT supposed to scan them):
    1. In Windows, right+click on the McAfee icon in Windows notification area (aka system tray), or start McAfee from start menu
    2. "On-Access Scan properties ..."
    3. "All processes", "Exclusions"
    4. "Exclusions ..."
    5. Add the "Thunderbird" and "Thunderbird Profile" folders and subfolders. Folder names must finish with a backslash. NOTE, if you have customized profile or account locations for mail, then you must exclude your custom locations.
  • Please write/call McAfee support to inform them of your difficulty. McAfee cannot fix what they do not know about.

If none of the above helps and you suspect McAfee is the cause then you may need to remove McAfee using the McAfee Consumer Products Removal tool, and seek different antivirus software.

If you are uncertain that McAfee is the cause, you can test whether something started during Thunderbird or Windows startup is involved:

If the problem is gone then you must dig to find which Windows installed program or Thunderbird addon is causing the problem.


  • The above changes disable various features, but you ARE still protected from viruses by McAfee's other protection features. (quote from McAfee) "you do not need to worry, as [when] the Real Time Scan is turned on [which it is by default] it will scan all the files and attachments and will alert you if there is any infection."
  • Common question: Thunderbird has worked fine for years, with antivirus (AV) product X. So isn't my new problem in a bug Thunderbird? Answer: No, typically not. Antivirus and firewall products are often tightly linked to little details in the software it is monitoring (eg Thunderbird), and which makes them susceptible to breakage or incompatibility from Thunderbird updates. It is the vendor of product X who bears responsibility to fix the problem in such cases.

Installation issue - If thunderbird update fails and you get a 7-Zip Access Denied error when downloading a new version or updating and you are using McAfee VirusScan Enterprise + AntiSpyware Enterprise 8.8, see the advice in this thread.

F-Prot Antivirus

When TLS and SSL protocols are used, e-mail scanning either cannot scan e-mails or may block them entirely. To solve this issue, reconfigure E-mail scanning to listen to the standard unencrypted port (110) or to any unused port Fprot support article here.

Thunderbird 3.0.1 was painfully slow for me on my 2.4 GHz core 2 duo until I took the following steps:

Disabled Global Indexing:

  1. Click “Tools” menu
  2. Click “Options...” menu item
  3. Click “Advanced” tab
    4) Uncheck “Enabled Global Search and Indexer”

Add a special exclusion to Antivirus software that covers the directory in the profile where TB stores its mailbox files. In my case the software was F-Prot so the steps were:

  1. Open F-Prot Interface
  2. Click “Virus Scanners”
  3. Click “Exclusions” under the advanced heading
  4. Click “Folder...”
  5. Type “C:\Users\$Username$\AppData\Roaming\Thunderbird”
  6. Check “Also include subfolders”
  7. Click “OK”

Note that the path to thunderbird's mailbox files varies with each OS, and exception instructions are specific to your virus scanner.

My theory is this: It looks like Thunderbird 3.0.0 and above generates a huge number of file open, seek or read events causing the virus scanner to scan the mailbox files over and over again. This only seems to affect scanners that ignore file extensions and scan all files. This could be proved if someone has some IO monitoring tools and can observe how TB interacts with the mailbox files.


Avast blocked and allowed apps

Most recent reports

Slowness and errors with nsemail.HTML

The release of Avast 10.3.2223 ( We think it is that version) has resulted in issues with avasts preventing Thunderbird from functioning correctly. The workaround appears to be to create an exception in avast for the file nsemail.html in your computers temp folder. I have to say "appears to be" as Avast have not yet replied to my email. [74], [75], [76], [77], [78], [79] SUMO links [80], [81]

Hangs with "Allow anti-virus clients to quarantine individual incoming messages"

As advised by user BigDave67 in th support topic 1094117 Now what seems to have worked for me is this:

  1. I disabled Avast.
  2. Disconnected from the internet.
  3. Started Thunderbird. (It didn't lockup.)
  4. Went to Tools > Options > Security > Anti-Virus
  5. Unchecked "Allow anti-virus clients to quarantine individual incoming messages" Clicked "OK".
  6. Reconnected to the internet.
  7. Clicked on "Get Messages" (It didn't lockup.)
  8. Closed Thunderbird.
  9. Enabled Avast.
  10. Restarted Thunderbird.

Avast 2015 causes invalid mail server certificate warnings sending/receiving mail

Avast describe the cause thus "The Mail Shield in Avast 2015 uses advanced scanning method for incoming and outgoing e-mails over SSL/TLS secured connections", the reality is they perform a man in the middle hack of your connection using SSL certificates by making themselves a top level certifying authority. As Thunderbird like all quality mail client maintains it's own certificate store the Avast hack does not work. There are instructions on the avast site explaining how to make then a trusted certifying authority.

Anecdotal evidence only, no definitive testing has been done. Love if it if somebody could find the time to do some thorough AVAST and TB testing!

Avast Blocks Sending and Receiving

My problem was Avast antivirus, once I stopped the service and restarted(virus program ect) thunderbird was once again working perfectly. Its a ssl cache problem within AVAST, not thunderbird. Learned from trying to eliminate avast from list of what may be interfering with thunderbird 3.1.2

AVAST blocks images in HTML email

"So I guess my problem's solved. I just wish I knew what happened. All I did was shut down the mail protection for a day, then restarted it. I didn't change a single setting in Mozilla Mail (didn't even open the options tab). Very strange, but I'm happy now!"

"Yeah, OK, so I uninstalled Avast, restarted the computer, launched TB and voila. Images are now loading when I click "Show Remote Content". "

AVAST blocks getting email


Examples: [82] From Thunderbird Support

  • Problem: Sending Mail Slow
  • Solution: Turn of outgoing mail scanning in BullGuard 15. Using instructions from the BullGard Support Site.

From MozillaZine Forum BullGuard Thread October 7, 2010:

  • Problem: "expanding or collapsing items (subfolders, accounts) in the Folders display often locks up TB completely and the progam goes into Not responding state. OS is Windows Vista Home Premium SP2"
  • Solution: Upgrade to Version 9: "Bullguard asked me to upgrade from Version 8 to the beta version of Verion 9 of their product. This does seem to have resolved the issue. I am now running with their Spamfilter 9.0 enabled and (fingers crossed) everything seems OK."

Norton Security

To quote a Norton support article "The email scanning feature in your Norton product cannot scan emails from the accounts that are configured for SSL. Email scanning can only scan emails from the accounts that are configured for POP3 and SMTP" Source page here This would mean that IMAP mail accounts are also not scanned, regardless of the connection security.

Norton's products have a number of exceptions, but SONAR is the one usually refered to. There are suggestions that you create a exception for Thunderbird to fix slowness. The Norton support forum has instructions here

Norton Security Deluxe is reported to cause issues with the Language reverting to English. [83]

The Global Search & Indexer

In regards to performance issues with antivirus software ... The global search & indexer normally only has impact during the initial indexing process. Performance of this feature should be acceptable after initial indexing is complete. The time needed for initial indexing normally varies from a few hours to a day, depending on the total size of all your messages.

The initial indexing process, performed upon upgrading from a prior version of Thunderbird, may be sped up by disabling on demand antivirus scanning of the Thunderbird mailbox files.

Bogus Windows System Proxies Left by Malware

Malware (usually found only on Windows systems, but could be on any system e.g. Mac or Linux) can install a bogus Internet access proxy. This proxy can slow down Thunderbird and other programs that access the Internet. Symptoms include "couldn't connect to proxy" errors, slow sending and receiving of email, and images not being displayed in HTML emails. Sometimes these can be found after a MalWareBytes scan.

Problem:Windows System Proxy blocking HTML images anecdote

  • don't think this is a MalwareBytes problem but you never know!

"FInally! Got the answer from support at AVIRA, which wouldn't update for me. (also, THunderbird wouldn't "connect to proxy").

I went to Internet Explorer -- tools-- internet options-- connections-- LAN settings. There, I unchecked "Use proxy server;..." Voila! it all works. Avira updates, Thunderbird shows images, Java updates. It all works again.

Apparently, when you do a full system scan with MalWareBytes, it puts that on for you to keep the computer from dialing out, but it doesn't uncheck it for you afterwards. How many of you use MalWareBytes? It's a good program, but check the LAN settings after scanning."

Solution: Disable Windows System Proxies

Internet Explorer | tools | internet options | connections | LAN settings and then uncheck "Use proxy server"

Solution for Spamfighter

After every upgrade you have to do the following. There seem to be two solutions from

  1. In Start Menu | Repair Spamfighter; when repair is finished restart computer and then Spamfighter Toolbar will be back!! OR
  2. more drastic solution:
    1. Close Thunderbird.
    2. Go to Start -> All Programs -> Fighters -> SPAMfighter -> Modify SPAMfighter.
    3. Click Next twice.
    4. Click on the box next to Toolbar for Mozilla Thunderbird and select Entire feature will be installed on local hard drive.
    5. Click Install and let the installation run.
    6. When you next open Thunderbird SPAMfighter will be back again

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