SMTP Username and Password should be optional
It's common to run an SMTP relay that doesn't require SASL authentication when accepting connections from trusted networks. Commento assumes SMTP isn't configured at all if you don't set the username and password. Those settings should be optional in SMTP config.
I was able to work around this using my Postfix relay by setting a bogus username and password in Commento, but I also had to use Postfix's smtpd_sasl_exceptions_networks
option to hide AUTH from Commento altogether. Doing that stopped it from trying to use the provided credentials.