The domain if the email address is used to determine the configuration (POP/IMAP and SMTP server names, SSL yes/no, authentication methods etc.), via several mechanisms:
# Config files on harddisk, in <installdir>/isp/*<i>emailaddressdomain</i>.xml.
#* Same file format as below.
#* This is only for intranet deployments, we won't be shipping any configs by default anymore, in favor of the following alternatives.