Conflicts with Postman plugin

SSH SFTP Update has probably implemented their own wp_mail function, disabling Postman SMTP. Only one plugin may implement wp_mail at a time.

Either that, or they are loading WordPress core files directly, which is dangerous.

You can tell which scenario it is by running the Postman diagnostic test and looking at the “wp_mail” owner. If it says pluggable.php, then it is the latter, and you have a security problem.