Any error messages from remote servers will be contained within our mail log; example (the server IP and system response key has been removed from the below example):
ECDHE-RSA-AES128-GCM-SHA256:128 CV=yes: SMTP error from remote mail server after end of data: 550-5.7.1 [systemip 7] Our system has detected that this message is\n550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail,\n550-5.7.1 this message has been blocked. Please visit\n550-5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError\n550 5.7.1 for more information. responsekey.176 - gsmtp
The above error links to the
Error Message page at Google. "Trusted" sites have a reverse DNS on their IP addresses (which must be done through your webhost, generally they don't delegate control over ip addresses to leased servers), and a DKIM or SPF policy (which I believe is default on most modern DNS configurations).
There is a vast array of items that could be going wrong, you could be blacklisted (
Blacklist Check), you may not be using a SPF or DKIM key on your DNS, or users could have reported your address as spam (like the above example message), or you may not have a reverse DNS entry for your server IP address (which would get your domain blacklisted through some overzealous services).
Also, if you're sending mail from one domain (mysite@gmail.com) on behalf of another domain (forum@example.com) gmail also may be dropping the messages as well for "masquerading" as another entity.
