SOLVED:
It's simply a question of spellings. In Plesk and in our mail clients mail user is written as
[email protected].
But testsaslauthd needs...
$ testsaslauthd -u <username without domain.tld> -r domain.tld -p 12345 -s smtp -f /var/spool/postfix/private/plesk_saslauthd
The related...