On my macosx laptop, sendmail worked out of the box.. but it finds the SMTP server from DNS. So at work it was using an internal SMTP server. And at home, it wasn't finding any SMTP server. And for some reason using the internal SMTP server seemed to only work for sending patches to internal addresses. I'm sure the built-in SMTP client could be configured to work as I needed, but I really had no desire to figure out how.
Following the instructions on Nishanth's post, I setup msmtp. (go macports!) And it works great. Somehow it automagically works properly both behind the proxy at work, and from home, depending on the systemwide proxy settings. (The power of Steve Jobs compels it!)
To send patches, I use this alias:
alias gsend='git send-email --from "<addr>" --envelope-sender "<addr>" --smtp-server /opt/local/bin/msmtp'
(replace <addr> with your email address)