Jul 3 22:12:17mailserver postfix/smtpd: fatal: no SASL authentication mechanisms
Jul 3 22:12:18mailserver postfix/master: warning: process /usr/libexec/postfix/smtpd pid 6195 exit status 1
Jul 3 22:12:18mailserver postfix/master: warning: /usr/libexec/postfix/smtpd: bad command startup -- throttling
This only ever happens in my experience when the authentication method is actually Dovecot. Usually the problem will be that Dovecot cannot start due to a misconfiguration and or permissions issue.
The solution is to try to trace through /var/log/maillog and find out what is wrong with Dovecot (assuming it cannot start).
After this postfix should work fine.
You should also enable debug info in dovecot.conf like this:
auth_debug = yes
auth_debug_passwords = yes
auth_verbose = yes
To give you an idea of what commonly goes wrong after a new Dovecot migration or install:
dovecot: auth: Error: passwd-file /etc/dovecot.passwd: open(/etc/dovecot.passwd) failed: Permission denied (euid=97(dovecot) egid=97(dovecot) missing +r perm: /etc/dovecot.passwd, euid is not dir owner)
chmod +x /etc/dovecot.passwd
dovecot: lda: Error: userdb lookup: connect(/var/run/dovecot/auth-userdb) failed: Connection refused
In this case you need to make sure /var/run/dovecot/auth-userdb and also /var/mail is set to the correct user id and group id or username/groupname as in your /etc/dovecot/dovecot.conf
first_valid_gid = 502
first_valid_uid = 501
last_valid_gid = 502
last_valid_uid = 501