Apologies if this is the wrong Stack Exchange site; moderators please advise and move this question if needed...
I have a website which I set up with Google Apps. Thus, I changed the DNS records (the MX records) to use Google's mail servers; in other words, when mail is received by my domain it is properly received by Google and ends up in the Google Apps mail app for my domain.
But almost every mail I send from my email which is received by a Gmail address (which is the vast majority of them), ends up in the spam box of the recipient. I am using my personal email account and taking advantage of the ability to send from multiple accounts, but I have set up the SMTP settings to use smtp.gmail.com with my custom domain email and not to just use my personal email as the forwarding email address. This, according to any Google Apps instructions I can find, is the correct SMTP server settings.
I am using this email as a support email for my Android apps; I can't afford for my replies to go in spam and my customers to assume that I didn't reply to them! This has happened several times now and each time I waited a day or two and then re-sent the mail from my personal email and got a response saying they never got the first email (from the Google Apps domain email address).
How in the world is Google flagging its own hosted email accounts as spam? They are simple plaintext messages, in fact they are always replies to messages sent explicitly to the email, so why in the world are they being marked as spam and what can I do about it??