Google Product Forums
Re: Email received by others sent by me from Google Apps has [SPAM] in the title line
bkennelly Apr 9, 2012 9:17 PM
Posted in group: Gmail

Categories: Ask a "how to" question : Composing and Sending Messages : Computer email program (please specify: Apple Mail, Outlook, Thunderbird, etc) :

Some filter does not trust your domain's mail yet, but you can help by defining an SPF record and enabling DKIM signing. SPF is used to identify the servers authorised to send mail for your domain, and DKIM is used to verify that the messages actually came from those servers.