Can't Share using Email

I’ve looked and I’m not finding a solution.

I have NextCloud installed on my VPS. It works great other than I can’t send e-mails to share files. It always has a pop-up error that says it could not find the email because either the server is unreachable or it uses a self-signed certificate.

I have a cert from Let’s Encrypt via Plesk. It’s working as well.

I also have email setup and testing sending email was successful.

Is this a cert issue? Or, is it something else I’m missing?

CentOS Linus 7.6.1810 (Core)
Plesk Onyx v17.8.11 (update #67)
PHP 7.2.22
Memory limit 512M