I have just spent the last few hours configuring my Ubuntu Hardy server to allow Modx to send mail.
I was interested in the Gmail via msmtprc solution but I eventually went with Postfix. Installation was pretty simple following these instructions
http://articles.slicehost.com/2008/7/29/postfix-installation
I left the php.ini mail settings as default, as Postfix takes over from sendmail once its installed.
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
; sendmail_path =
After installation and reboot i was able to send mails via the eForm snippet.
note: I am only using it to send mails via eForm and have not tested it for anything else.