Google Apps allows for custom SMTP serversI've been using my google account to store mail for a while now, including forwarded mail from client organisations, where this is allowed. The only problem that I've had so far is that when I'm sending mail as another identity (like a client one), it would always come up as "from email@example.com on behalf of firstname.lastname@example.org" Its a bit annoying, but thats what Google had to do in order to be good email citizens and not get everything marked as spam. As it turned out, many Exchange servers automatically mark such email as spam anyway, so some people lost mail that I sent them.
Google recently introduced a new feature, whereby you can specify your own SMTP server to use when sending mail, and you can specify a different one for each identity. This is great, as you no longer get the on behalf of bullshit, and your mail doesn't get spammed any more. I was originally stumped by the fact that I couldn't get to the SMTP servers of the organisations that I wanted to send mail from, but then it dawned on me that I could use any SMTP server that would allow me to relay mail as other identities. I used my ADSL provider and all is good. Of course, if the organisations that I'm sending mail as had SPF rules set up to disallow this sort of stuff I'd still be in trouble, but so far that hasn't been a problem.