I can't say for freenet.de, and I doubt somebody will explain his antispam tactics in details, but generally using 4xx like this is very common.
1. Graylist filter checks if you re-send the same message, because many spammers don't even try. 2. It may be result of ratelimiting, message above ratelimit may get 4xx 3. It may be attempt to detect the suspicious mass-mailing. Some logic in antispam detects new template or some rule marks message as suspicious and delays it to understand if it was a single message or is a part of mass-mailing campaign. A single message later may be allowed to pass. 4. "Same" (according to e.g. ML or some unreliable heuristics) messages were marked as spam by users and are awaiting for additional analysis or manual rules/actions. 5. There are some temp failures e.g. DNS timeouts on name resolutions for MX / PTR / SPF / etc and/or reputation data is insufficient and lot more. Just do the things as you expected to and implement reasonable retry strategy. 15.11.2018 15:25, Stefan Bauer пишет: > freenet.de rejecting with 451 that should be a perm error (suspicious > message) > > Hi, > > > we just noticed that freenet.de is reject some of our mails with > > > 451 Suspicious message, please come back later (in reply to end of > DATA command) > > > That makes no sense to us. If mail looks fishy to freenet, they should > generate a perm. error (5xx) and not let us try over and over again. > > > Can anyone from freenet please clarify? > > > Thank you. > > > Stefan > > > > _______________________________________________ > mailop mailing list > [email protected] > https://chilli.nosignal.org/cgi-bin/mailman/listinfo/mailop -- Vladimir Dubrovin @Mail.Ru
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