I feel like such a newbie, but I gotta ask... I've got a Linux box acting as our work email server. I have a home user with a static IP address. This home user cannot open his email client and send email using my email server as the SMTP server because his connection to the Internet is an IP address that my server will not accept. I thought this was a hosts.allow issue, but cannot seem to get it working no matter what I do. I know it's got to be something simple, but I'm obviously missing it. Please give me some direction. Thx!

Welcome to LQ.

It could be a firewall issue:
iptables -L

If the machine itself isn't connected to the net you amy need to forward port 25 from the device with the public ip to the address of the mail server.

what you can do is in /etc/mail/access

put the ip, and relay, or accept

Then do makemap hash /etc/mail/access.db < /etc/mail/access

Then your mail server will know what to do when it gets mail from him..... THIS ONLY WORKS IF YOUR USERS EMAIL ADDRESS IS NOT FROM YOUR DOMAIN.....

I think you need to have your remote user go over his email settings.... as long as he has an account on your mail server, then sendmail really shouldn't care what IP he is connecting with, its the user name and authentication that'll let him in.

Reading over the replies I discovered my original post was not a thourough description of the problem. My home user has an Internet connection through COMCAST.NET. He takes his work laptop home and checks his work email fine (ex. work email = WORKEMAIL.COM) and he can reply to anyone within that same domain (ex. joe@workemail.com & susan@workemail.com) using his normal work email settings. The problem arises when he recieves and email from someone outside of the work domain and tries to reply to them (ex. jim@somewhereelse.com) because our email server does not allow relaying. Hope that makes better sense. I've not quite worked through the first replies, but I hope I made the question clearer for future replies. Thx again!

Take a look at:
/etc/mail/access

You can setup a relay for the users IP in there.

Looks like /etc/mail/access was the fix. I did, however, run into another problem. His IP is not static, it's "persistent"... meaning it stays the same for a few months. So, rather than changing the relayed IP in the future can I just place his email address in the file with RELAY next to it? I planned to try this, but is that going to work?

We accidentally did the same, and it worked. We had a line:
foo.bar RELAY
in /etc/mail/access, and some notebooks configured to be in domain foo.bar can send mail from here and from home, too.

Strangely enough, so far no spammers tried to relay though our mail server pretending he sends his mail from our domain - though that would be a handy idea.

Based on my experiences, I think one can allow relaying based on domain name, but things may change...

I wonder if it is possible to specify an IP address range in /etc/mail/access? It would be still better.

Yes you can use ip addresses. Although I haven't tried it I would anticipate dynamic dns addresess working equally well too.