relayhost without authentication (easy fix)
Setting up a relayhost without username/password results in "smtp_sasl_auth_enable = yes", which means it is trying to authenticate to the relayhost, but cannot (nothing to authenticate with), hence a SASL authentication failure error from the relayhost.
No relayhost credentials provided shouldn't cause authentication to be tried.
Server OS: Debian Server OS version: stretch (9.6) ISPConfig version: 3.1.13
When the username field is empty, do not flip the smtp_sasl_auth_enable setting, but leave it instead to its default of "smtp_sasl_auth_enable = no".