One reality of the Internet these days is the virtual hosting of multiple domains on the same server. This works fine for Web service. A single server fakes many virtual servers.
For electronic mail, sendmail has the ability to
accept email for multiple domains. Generally, all those domains
are aliases (see Aliases for your system in basic
sendmail configuration) for your system. If your system
has the following aliases:
then user@domain1.com and user@domain2.com point to the same user account and email folder. This may, sometimes, be wrong. Complex user routing can be understood this way:
If you receive email for one_user@one_domain then forward this mail to another_user@another_domain using another_email_server.