In the past weeks we had the problem, that some of our emails sent from our company SMTP server were flagged as Spam by Gmail (and some servers that use a spam filter from google), although we didn’t use any language we considered typical for spam.
After some investigation and fiddling with the server settings, I noticed we were using IPv6 to connect to the Gmail SMTP server. I found some other posts indicating that Gmail has problems finding the reverse DNS entry if you connect via IPv6, thus flagging your emails as spam.
There seem to se some workarounds for this:
- If you don’t use IPv6, you can simply disable IPv6 in your MTA (Mail Transfer Agent), so only IPv4 is used (where Gmail can resolve the reverse DNS entry). For postfix you can do this with:
main.cf12# Disable IPv6inet_protocols = ipv4
- You can create a transport map for gmail (see http://tanguy.ortolo.eu/blog/article109/google-ipv6-smtp-restrictions for postfix)
- You can set up a reverse DNS entries for all your IPv6 addresses (see https://yeri.be/gmail-google-apps-ipv6)
After we configured our MTA to only use IPv4 when connecting to Gmail, the problems were immediatly gone and our email are now delived as expected.