So I used to use the little tip found here http://forums.macosxhints.com/showthread.php?t=51901 to send email from my school account through my gmail account by simply changing the "Email address" field in Mail > Preferences > Accounts > Gmail Account, but now I was looking to do the same thing with my .Mac account. I've set my school account to forward to my gmail account which forwards to my .Mac account. So school -> gmail -> .Mac. This way school comes to gmail (an archive of sorts for the massive amount of space given) and then to .Mac so I can use it synchronized within Mail.app and .Mac webmail. When I try to simply input email@example.com in the "Email address" field when trying to send mail it gives me the error: Cannot send message using the server (null) The server response was: 5.1.0 <firstname.lastname@example.org>... From address does not match authentication Use the pop-up menu below to try a different outgoing mail server... However, when I change the smtp to a gmail smtp and the from address to my school address, it is able to send and gets routed to the gmail inbox, but not all the way to the .Mac inbox. A very frustrating 3/4 solution to my problem. I've looked everywhere for help on how to do this. Apple's .Mac aliases in their web mail preferences don't help, since those can only be usd for @mac.com addresses. Also since the email is already being forwarded to my .Mac account, I don't want to have to set up a different account in the preferences pane. Like this hint recommends I do, http://email.about.com/od/macosxmailtips/qt/et061402.htm This page seems to imply that it's not possible. Mainly the second comment, http://www.macosxhints.com/article.php?story=2006110705365729 This one mentions an alternate IMAP account, but that won't work either. http://www.macosxhints.com/article.php?story=20041012201406259#comments http://forums.macrumors.com/showthread.php?t=112689&highlight=mail.app+.mac+addresses was all I could find here on MacRumors about it and the last comment there implies that it isn't possible either. Apple really needs to work on this if a solution to a simple problem like this is really this complicated. Gmail among others has the option built right into their preferences to allow users to send email from any address that can be verified. .Mac and Apple really need to fix this.