Email rejected by relay server


i setup email with my isp smtp as relay server.
when tested with few test email, everything worked fine.
now testing few more users with real email conversations, the isp smtp now rejecting all my email as spam with error:

host xxxx said: 554 5.7.1 Message refused by Heuristic check. This email has been rejected. The email message was detected as spam. (in reply to end of DATA command)

is there anything in data command that can trigger spam detection?
anyway to avoid this?


NethServer Version: 7rc3

Maybe relay is not required!

Does the ISP need SMTP authentication? The “smarthost” feature could help:

sorry i didnt know smarthost vs relay.

i think it’s smarthost that i setup because i only use NS gui and the smarthost do not need authentication.

also, after further test, not all email is blocked.
only certain email. i’m trying to find out if there is something in the email that caused it get blocked
because many email gets through without problem.

any idea what/where to start troubleshooting?

I have encountered this in customer’s locations using multi-function copiers to scan to email. What was flagging the emails for us turned out to be the subject line of the email, or in some cases lack of a subject.
Although in one instance the customer was using residential class internet in a business environment and their ISP was limiting the amount of emails that could be sent within a certain period of time.