Sending mail fails if using SMTP with authentication.
Using XOOPS 2.0.13.2 you get the error "SMTP authentication fails" whenever XOOPS tries to send an e-mail via SMTP with authentication.
I looked around and the problem is common although I couln't find a straight forward answer, which is: update phpmailer from 1.65 (included in XOOPS 2.0.13.2 distribution) to version 1.73 available on http://phpmailer.sourceforge.net.
Thanks!

If your admin mail is same as SMTPAUTH‘s name，maybe it can run.

https://xoops.org.cn/modules/newbb/viewtopic.php?post_id=24957

BTW：how to update phpmailer？
Just replace class.phpmailer.php and class.smtp.php?

You-re right, in order to update to update phpmailer in XOOPS you just have to replace the files class.phpmailer.php and class.smtp.php.