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, similar to the legacy rdf files in <installdir>/isp/*.xml. This is only for intranet deployments, like we have for Google Mail right now. (This may won't be dropped shipping them anymore, in favor of the next stepsfollowing alternatives.)
# Try to contact a mail configuration server of the provider
#* 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]].