working locally not externally, how to check what failed?

[Voytek Eymont]

this morning I noticed not getting any email on remote to server mail
client, though, in webmail/squirellmail on mail host there were new emails

restarting with doveadm stop/dovecot 'fixed it'

what could've gone wrong, what to look for in the logs ?
thanks for any pointers

long running dovecot/postfix/mysql setup

dovecot --version
2.3.14 (cee3cbc0d)