I have a client that says they are getting this error message repeatedly.

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'jpavek@exhibitormagazine.com'. Subject 'Re: EXhibitor Mag's online Buyers Guide', Account: 'mail.digitaloutrage.com', Server: 'mail.digitaloutrage.com', Protocol: SMTP, Server Response: '550 5.7.1 <jpavek@exhibitormagazine.com>... Relaying denied. Please check your mail first or restart your mail session.', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

Know what this means and how I can test on my end to make sure I have everything right.

It sounds like they are trying to send email before checking it. ie. writing emails in Outlook or other client and then connecting to send & receive. Normally once the server authenticates the user they will be able to send the mail (my logs are full of this!)

Is there a setting for them to change so it automatically pops before sending?

As far as I know there isn't. Basically they can ignore the message and just do send and receive a second time, or lne up messages without hitting send , then send after the intial send and receive (what an awkward sentence - sorry!)

There might be some other 'magical' solution that does not compromise the box, but I'm not aware of it...

Do you know if there is a setting I can change on the raq without opening up my mail server to any security issues?

I think you must install POP-BEFORE-SMTP module

Set relay window for hm 20 min or so in the CP/ Services /Relay Window