https://www.emailsecuritycheck.net/index.html

https://www.mail-tester.com

http://www.isnotspam.com/

https://glockapps.com/

https://senderscore.org

http://www.anti-abuse.org

https://sendersupport.olc.protection.outlook.com/snds/data.aspx?wa=wsignin1.0 - MS SNDS

https://www.wirehive.com/thoughts/5-top-tips-reviewing-postfix-mail-queue/

http://freesource.info/wiki/Dokumentacija/Postfix/antispam/restrictions

Postfix SMTP relay and access control

https://major.io/2013/04/14/remove-sensitive-information-from-email-headers-with-postfix/

https://www.linuxcnf.com/2018/11/how-to-change-default-mta-in-cent-os-7.html

https://wiki2.dovecot.org/HowTo/PostfixDovecotLMTP\

https://wiki.dovecot.org/MailboxSettings

https://wiki.dovecot.org/Namespaces

https://wiki2.dovecot.org/HowTo/AntispamWithSieve

https://wiki2.dovecot.org/Pigeonhole/ManageSieve/Configuration

https://wiki2.dovecot.org/Pigeonhole/Sieve/Examples

https://wiki.dovecot.org/Migration/Dsync

https://wiki.dovecot.org/PerformanceTuning

https://workaround.org/ispmail/stretch/filtering-out-spam-with-rspamd

Bypass check for specific address

https://www.dcc-servers.net/dcc/

https://mailutils.org/manual/html_chapter/Sieve-Language.html

https://p5r.uk/blog/2011/sieve-tutorial.html

[Dovecot] Sieve mark message as read

http://sieve.info/clients

https://github.com/thsmi/sieve/blob/master/nightly/0.2.3/sieve-0.2.3k.xpi

https://www.rudraraj.net/how-to-install-roundcube-with-nginx/

https://wiki.dieg.info/postfix_backup_mx

https://www.vimbadmin.net/

https://sourceforge.net/projects/postfixadmin

https://sourceforge.net/p/postfixadmin/wiki/Regex_aliases/

https://blog.returnpath.com/how-to-explain-authenticated-received-chain-arc-in-plain-english-2/

HowTo: DMARC

http://diesel594.blogspot.com/2013/12/convert-imap-folder-names-to-readable.html

Send mail from the command line with an external SMTP server on Linux

https://serverfault.com/questions/846026/can-i-send-a-mail-through-gmail-with-swaks

https://thomas-leister.de/en/mailserver-debian-stretch/

https://www.c0ffee.net/blog/mail-server-guide

PLAIN - one Base64 message after auth plain command perl -MMIME::Base64 -e 'print encode_base64(«\0user\@domain.com\0password»);' \0 (Null character) is required as a delimiter

LOGIN - two Base64 messages user and password perl -MMIME::Base64 -e 'print encode_base64(«user@domain.com»);' perl -MMIME::Base64 -e 'print encode_base64(«password»);'

described here - https://www.fehcom.de/qmail/smtpauth.html

openssl s_client -starttls smtp -connect server:port openssl s_client -tls1_2 -connect server:port

Spamhaus test - https://blt.spamhaus.com