[CentOS] Postfix vs. Thunderbird on Mac OS

Hi,

I'm running our local school's mail server on CentOS 7, Postfix and
Dovecot. We get quite a lot of spam, so I have the following sender
restrictions in my /etc/postfix/main.cf:

--8<------------------------------------------------------
# Restrictions SMTP
smtpd_helo_restrictions = reject_unknown_helo_hostname
smtpd_sender_restrictions = reject_unknown_sender_domain,
  check_sender_access hash:/etc/postfix/sender_access
smtpd_recipient_restrictions = permit_mynetworks,
  permit_sasl_authenticated,
  reject_rbl_client zen.spamhaus.org,
  reject_rhsbl_reverse_client dbl.spamhaus.org,
  reject_rhsbl_helo dbl.spamhaus.org,
  reject_rhsbl_sender dbl.spamhaus.org
smtpd_relay_restrictions = permit_mynetworks,
  permit_sasl_authenticated,
  reject_unauth_destination
smtpd_data_restrictions = reject_unauth_pipelining
--8<------------------------------------------------------

Most folks are using Thunderbird on Linux, and everything works
perfectly. One single user has a MacBook Air with Thunderbird on Mac OS
Mojave, and her outgoing mails are rejected with the following error
message in /var/log/maillog on the server:

--8<------------------------------------------------------
Sep 16 14:22:32 sd-48011 postfix/smtps/smtpd[14434]: NOQUEUE: reject:
RCPT from villa.figaret.pck.nerim.net[62.212.106.47]: 450 4.7.1
<Air-de-bea.scholae.lan>: Helo command rejected: Host not found;
from=<xxxxx.yyyyyyyy at scholae.fr> to=<info at microlinux.fr> proto=ESMTP
helo=<Air-de-bea.scholae.lan>
--8<------------------------------------------------------

As far as I understand, it has to do with this MacBook's host
configuration.

Any suggestions?

Cheers from the sunny South of France,

Niki Kovacs
-- 
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