FortiMail unit replies with a TEMPFAIL SMTP reply code

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The FortiMail unit replies with a TEMPFAIL SMTP reply code, and the event log shows "Milter (fas_milter): timeout before data read".
The timeout is caused by the FortiMail unit not responding within 4 minutes.
Slow or unresponsive DNS server response for DNSBL and SURBL scans can cause the FortiMail unit's antispam scans to be unable to complete before the timeout. When this occurs, the FortiMail unit will report a temporary failure. In most cases, the sending MTA will retry delivery later. If this problem is persistent, verify connectivity with your DNSBL and SURBL servers, and consider providing private DNSBL/SURBL servers on your local network.