This is getting ridiculous now. A client sent me an email from his own email domain (hosted by google), and google rejected the email I replied to him.
I seriously think that google is doing this on purposely so that people can use their server for emails.
Here is the reply I received from google
This is the mail system at host server.mydomain.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<client@kclient.com>: host ASPMX.L.GOOGLE.COM[172.217.222.26] said: 550-5.7.1
[aaa.aaa.aaa.aaa 12] Our system has detected that this message is
550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to
Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1
https://support.google.com/mail/?p=UnsolicitedMessageError 550 5.7.1 for
more information. l11si7502380vsj.137 - gsmtp (in reply to end of DATA
command)
I have no issues sending email to other clients. The problem is only with google/gmail.
A few weeks ago, all email sent to google/gmail got rejected and reason is "likely suspicious due to the very low reputation of the 550-5.7.1 sending domain", then it worked, then starting a couple days ago, all emails sent to google got rejected as "unsolicited mail".
We never sent any spam emails to our customers, we have configured DKIM, SPF, DMARC etc, and mail-tester.com result is 10/10.
Reported to gmail (there was a form to submit), of course no reply.
If anyone knows how to fix this, please let me know. Thanks!