If your server is doing any of these things, then you appear to be sending spam. As a result, Gmail may block your server’s IP address.
Research by ReturnPath, our investigations and other email delivery experts, suggests that Google may use signals from these public blacklists:
pbl.spamhaus.org – This blacklist includes dynamic and non mail server IP ranges. For a server to be listed in here would be unusual.
sbl.spamhaus.org – This blacklist includes emails that Spamhaus has identified as spam.
xbl.spamhaus.org – This blacklist includes bots and exploit agents.
cbl.abuseat.org – This blacklist includes emails sent to spam traps or reported by their users as spam.
You can use the Multi-RBL lookup tool to check these and other lists. Inclusion on these lists is a good indication that you have some type of spam issue on your server.