Done that way, email leaving the server is sent "from"
user@mail.domain.com, rather than
user@domain.com. I don't know anyone who _wants_ their email to be sent from
user@mail.domain.com, even if the name of the server actually is mail.domain.com, or just mail.
Personally, my mail servers use names like Garfield and Odie, although even if I used stale names like mail and mail2 I would not want my outgoing mail to be addressed from anything but
user@domain.com, unless there were multiple subdomains like sales.company.com, engineering.company.com, et. al., and even then the email addresses should be
user@sales.company.com and
user@engineering.company.com if anything other than
user@company.com.