> Maybe the MTAs that such senders use are so customised as to be capable > of only sending, not receiving, mail?
Usually, yes, these systems are typically decoupled from the firms' "regular" emailing infrastructure (maintained by different business units etc.) and aren't interested in receiving emails: Bulk email often doesn't care about replies, and tends to be sent on behalf of external clients, who are in the Reply-To/From header /if/ they care about getting replies. Those headers are also often completely spoofed for spam emails, so testing if /that/ email server is reachable on port 25 doesn't tell you much either. -- Mit freundlichen Grüßen, / Best Regards, Sven Schwedas, Systemadministrator ✉ sven.schwe...@tao.at | ☎ +43 680 301 7167 TAO Digital | Teil der TAO Beratungs- & Management GmbH Lendplatz 45 | FN 213999f/Klagenfurt, FB-Gericht Villach A8020 Graz | https://www.tao-digital.at
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