Exim

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A message that you sent could not be delivered to one or more of its
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  perez******@gmail.com
    SMTP error from remote mail server after end of data:
    host gmail-smtp-in.l.google.com [2a00:1450:400c:c06::1a]:
    550-5.7.1 [2001:41d0:52:400::a67] Our system has detected that this message does
    550-5.7.1 not meet IPv6 sending guidelines regarding PTR records and
    550-5.7.1 authentication. Please review
    550-5.7.1  https://support.google.com/mail/?p=ipv6_authentication_error for more
    550 5.7.1 information. a125si20128233wmf.3 - gsmtp

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nano /etc/exim4/exim4.conf.template

SPAMASSASSIN = yes
SPAM_SCORE = 50
CLAMD =  yes

disable_ipv6 = true

domainlist local_domains = dsearch;/etc/exim4/domains/