I’ve seen a couple of threads post the same problem, WP SMTP plugin giving error that the SSL is not valid.
I had that problem today and the solution was deleting WP SMTP and installing Post SMTP plugin, then I tried all kinds of combinations in the configurations. And had success with the following:
Outgoing Mail Server Hostname - mail.yourdomain.tld
Outgoing Mail Server Port - 587
Security - STARTTLS
Authentication - Plain
So, hope it helps, couldn’t find a reply on the forums.