Conflict with Post SMTP

After installing Pareto, our mail solution based upon Post SMTP started displaying the following notice:

Postman: wp_mail has been declared by another plugin or theme, so you won’t be able to use Postman until the conflict is resolved.

There was a (experimental) workaround suggested by Post SMTP for cases like this:

I highly recommend to change and TEST Post SMTP with the value PHPMailer.
if it will not work properly you can change back to the default value: PostSMTP.

Unfortunately, the workaround didn’t make the notice go away. Please advise how to make both plugins work together with no side-effects. I can’t test Pareto right now in a live environment because of this issue.

Thank you.