Gmail Blacklist Criteria
Gmail does not disclose the details of their filtering process. If they did, spammers would quickly find a way around their filters.
However, we’ve learned about some common reasons why Gmail may reject email from your server.
The most common reasons are:
Sending large volumes of email.
Sudden changes in email volume.
Sending email to “spam trap” addresses.
Sending email to unknown users.
Inclusion of your server’s IP on public blacklist.
Gmail users marking your messages as spam.
Using a new IP address to send email.
Incorrect DNS Settings