I had the same thing happen. Emails have been working fine when using the gmail smtp server for months. Then a few days ago it stopped working with the same error as mentioned above. I have a wordpress site on the same server as nodebb with the same relay settings and it continues to work fine. It also works fine when using telnet from that same server. NodeBB is the only thing that seems to be effected. I solved the problem by disabling "Use an external email server to send emails" and just letting postfix relay the mail for me. Just thought I'd share.