We have been using our SMTP server to send emails to our clients for a few years, all of a sudden, gmail rejected them all. We have not changed anything, server is not compromised, and we don't send spam emails. We only send about 100-200 emails to our clients per day.
I checked spf, dkim, dmarc etc, all look good.
So how to fix this?
Google has a postmaster tool, I have verified the domain and the smtp server.
Strange thing is, it started 3 days ago. About 2 hours ago, I was able to send an test email to my own personal gmail account (it resulted in spam, and I clicked "report not spam"). Now when I try it, it got rejected again.
This is the message I am getting
<mymaskedpersonalemail@gmail.com>: host gmail-smtp-in.l.google.com[2607:f8b0:4023:1407::1b]
said: 550-5.7.1 [2607:5300:60:47d6:: 19] Our system has detected that
this 550-5.7.1 message is likely suspicious due to the very low reputation
of the 550-5.7.1 sending domain. To best protect our users from spam, the
message has 550-5.7.1 been blocked. Please visit 550 5.7.1
https://support.google.com/mail/answer/188131 for more information.
e10si145705ilc.155 - gsmtp (in reply to end of DATA command)
Thanks
