Sendmail - relay from unauthorised ip - ideas?

Problem: We use our mail server for both sending and receiving (as opposed to using ISP provided mailserver for sending). Mail server is remotely based, it's configured to only accept mail for relaying from the office IP (fixed IP, DSL connection). Someone at work wants to be able to send mail when at home - where they use their own DSL connection. Unfortunately their home connection is dhcp based dsl - so I can't do the easy option of just adding another IP to the allowed relaying list.

Things I've considered: Tell colleague to set up two mail accounts on their laptop - one for work, for home - using their home ISP provided mail server. Not really a viable option - colleague is not at all technical, they will screw it up. Set up a vpn, let colleague connect to that from home? A lot of hassle and potential security issues just so they can send email...

Anyone got a nice, easy answer?

Why not use SMTP auth? You configure your mail server to allow relaying for authenticated clients, then in the road warrior's email software you setup to use SMTP AUTH.

Obviously the specifics depends on your mail server and client. But we use sendmail and various email clients and it all works fine. If you're using sendmail, search for smtp auth on sendmail.org

-c