Hi,
Your fundamental problem is that you are (presumably) sending email from user@hisdomain.com (your user's email address) via someotherdomain.com (your smtp server's domain defined in system settings). This is called relayed mail and many mail administrators and automated spam filters will flag this as spam. While not all relayed mail is spam, pretty much all spam is relayed mail.
I use a reliable smtp service (JangoSMTP) and sometimes my users have to white list the server. One other problem is when one user (joe@domain.com) sends to another user at the same domain (jane@domain.com) the message will be blocked because it would not seem sensible to receive relayed mail within the same domain. This can also be resolved with white-listing.
For the GMail user whose mail is ending up in the spam bucket, GMail provides a simple (Not Spam) button. I believe that it will probably spam every unique user from the system. If there's a way to white-list a domain in GMail, I don't know it.
But you can reject the idea that this is AwareIM related.
You can avoid this if each user provides their own outgoing smtp server (the gmail user would apply standard GMail outgoing server properties). In the case where your application provides an Account layer, you can do it at that level and this will be more practical.
Hope this helps.
