Over the weekend I was plagued with Gmail bouncing messages. The message was
SMTP error from remote mail server after end of data: [IP address] Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. q91si6161949pja.149 - gsmtp
Eventually I worked out that the message included links to tinyurl.com and this is a url shortening service which presumably Gmail doesn’t like because the eventual destination of the link is hidden. When I changed the links to the final destination the emails went through. Mind, I had to delete lines from the usermessage table so the message was sent again to the gmail addresses when I requeued it.