That would give you a debug log of the connection which may show the cause.
This could be a change in SSL handling like a missing certificate, a server which does not support TLS v1.2 or simply a little protocol change in FM15 which the server doesn't like.
Using SMTP via CURL functions. Check SendMail functions to build email with/without HTML, multiple attachments and properly encoded names & texts. Tutorial: Sending emails in FileMaker with MBS Plugin
While we have used your plugin in the past and love it, I believe that this is something that should work out of the box without the need for a plugin.
I'm sure it would work but will only go down that route if absolutely necessary.