SMTP error from remote mail server after end of data:
host gmail-smtp-in.l.google.com [2a00:1450:4013:c01::1b]:
550-5.7.1 [2a00:1178:1:43:8000:0:ebc0:46bc] Our system has detected that this
550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records
550-5.7.1 and authentication. Please review
550-5.7.1
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550 5.7.1 information. u131si11465531wmu.18 - gsmtp