FortiMail
FortiMail provides advanced, multi-layer protection against the full spectrum of email-borne threats
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Article Id 195878
Article

When sender reputation or MSISDN reputation is enabled in a session profile, it relies on the thresholds set for "Temporary fail client at" and "Reject client at" to determine if it should block an SMTP client or not. (For MSISDN reputation, this threshold is instead located in the field "Auto blacklist score trigger value".)

If either of these two values have a low threshold, a legitimate SMTP client's sender reputation score can easily reach that threshold and the FortiMail unit could refuse further connections from that SMTP client. If this is a common problem, you can increase the value of the thresholds in the session profile (located under Profile > Session).

However, to prevent the problem from recurring, you should locate and resolve the reason that the sender reputation score is increasing: it may be due to an increased amount of spam originating, for example, from a compromised MTA.