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Here is the scenario. Maybe you’ve just gotten a bounce message that looks like this:Aug 25 11:20:24 s1 postfix/smtp[26906]: 98299221BB: to=, relay=gmail-smtp-in.l.google.com[142.250.141.27]:25, delay=1.2, delays=0.04/0.77/0.2/0.21, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[142.250.141.27] said: 550-5.7.1 [206.125.175.2] Our system has detected that this message is not RFC 550-5.7.1 5322 compliant: duplicate headers. To reduce the amount of spam sent 550-5.7.1 to Gmail, this message has been blocked. Please review 550 5.7.1 RFC 5322 specifications for more information. j6-20020a637a46000000b0042b3a763e76si3563504pgn.127 – gsmtp (in reply to end of DATA command))Or perhaps it looks like this:Aug 25 12:48:59 s1 postfix/smtp[14180]: C90492056B: to=, relay=aspmx.l.google.com[142.250.141.27]:25, delay=0.69, delays=0.08/0/0.39/0.22, dsn=5.7.1, status=bounced (host aspmx.l.google.com[142.250.141.27] said: 550-5.7.1 [206.125.175.2] Our system has detected that this message is not RFC 550-5.7.1 5322 compliant: 550-5.7.1 Multiple ‘From’ headers found. 550-5.7.1 To reduce the amount of spam sent to Gmail, this message has been 550-5.7.1 blocked. Please visit 550-5.7.1 https://support.google.com/mail/?p=RfcMessageNonCompliant 550 5.7.1 and review RFC 5322 specifications for more information.