Forum OpenACS Q&A: Response to What modifications does mail need for a dynamic ip address?

Try setting up your mail delivery agent so it drops all outgoing mail
on your ISP's SMTP server for relaying.

I don't know how to do that with sendmail or qmail but it should be
pretty straightforward. You could probably do it by setting up a proxy
on your machine port 25 which connects to smtp.yourisp.com