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/*.xml. Same file format as below. This is only for intranet deployments, we won't be shipping them any configs by default anymore, in favor of the following alternatives.
# Try to contact a mail configuration server of the provider
#* (Not yet implemented) Define a DNS TXT record on domain example.net (for my.account@example.net) which contains an URL like https://mailconfig.example.net/mozilla.xml . A similar, but slightly different proposal is described in detail on [[Thunderbird:Autoconfiguration:DNSBasedLookup|DNSBasedLookup]].