Hello again,
I want to thank everyone for all their help getting me this far -- the mail server system is almost completely functional. I talked with the ISP (technically I have to talk to someone who talks to them) and got them to register the MX record for mail.domain.com to be associated with the static IP of the mail server.
I noticed that I can send emails from the mail server from
webmaster@domain.com and they'll show up in my gmail account as such, but the mail server can only receive an email if it is sent to
webmaster@mail.domain.com, not
webmaster@domain.com. I am not sure if this relates to the MX record being set up for mail.domain.com or the server hostname being mail.domain.com or something else entirely.
The email address the message shows up from when I check in gmail is
webmaster@domain.com but if you reply to it nothing happens. If you change it to include the mail part, it works fine.
I appreciate your feedback. Thank you.