How to Telnet to an SMTP Server
https://drewsymo.com/2016/05/09/how-to-telnet-to-an-smtp-server/
Как перенести почту на другой сервер или другую платформу? Imapsync!
imapsync - Email IMAP tool for syncing, copying and migrating email mailboxes.
The imapsync command synchronises mailboxes between two imap servers.
OpenSSL выручает.
Пробуем
# openssl s_client -connect mail.foobar.com:465
получаем ошибку Verify return code: 18 (self signed certificate)
Почему так происходит - https://serverfault.com/questions/726262/ssl-certificate-self-signed-instead-of-valid
You might need to add -servername example2.com in your OpenSSL command. Unlike browsers, the s_client doesn't send SNI information by default, I believe, and your web server might be choosing which certificate to return based on SNI.
Рабочий вариант
# openssl s_client -connect mail.foobar.com:465 -servername mail.foobar.com ... Verify return code: 0 (ok)
Добавляем домен в специальный postmaster сервис и изучаем
https://rtcamp.com/tutorials/mail/server/port-numbers/
Смотрим какие порты используют Postfix (master), Exim и dovecot
# netstat -tulpn | egrep 'master|exim|dovecot'
Смотрим какие процессы работают на указанных портах
# netstat -tulpn | egrep '25|465|587|110|143'
dovecot: master: Error: systemd listens on port 143, but it’s not configured in Dovecot. Closing.
Добавить в /etc/postfix/main.cf
inet_protocols = ipv4
Добавить в /etc/postfix/main.cf
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
Скрипт для починки
# service postfix stop; rm /var/lock/subsys/postfix; rm /var/spool/postfix/pid/master.pid; fuser -k /var/lib/postfix/master.lock; sleep 5; service postfix start; sleep 5; service postifix status
RBL
Виноват greylisting.
https://wiki.apache.org/spamassassin/Ham
What does the word "Ham" mean, in the context of anti-spam?
Nowadays, it's likely that everyone knows what Spam means, in the context of e-mail. The use of the word "Ham", on the other hand, is relatively new and sometimes confusing.
"Ham" is e-mail that is not Spam. In other words, "non-spam", or "good mail". It should be considered a shorter, snappier synonym for "non-spam".
Its usage is particularly common among anti-spam software developers, and not widely known elsewhere; in general it is probably better to use the term "non-spam", instead.