Can only receive first test email, then no more sending

[Guido1982]

It worked for me either with or without setting the "from" field, after I discovered the SMTP user/account had to be the main user on the DirectAdmin panel. I was using a sub-user earlier. I think we're still running PHP 5.5 on this server. Anyway, I've successfully sent mail to, for instance my hotmail account (sometimes in the spamfolder, sometimes directly into the inbox), which at first wasn't working at all.

You mean, 'not validated' on the SSL or TLS options?