SMTP Connection stopped working
If the same credentials work on one server and not another, there's a good chance that you have php-mcrypt installed on one and not the other.  When php-mcrypt is installed, CiviCRM encrypts the SMTP password, and assumes that any passwords are encrypted.  This can be a problem if your PHP environment changed (in particular, php-mcrypt is deprecated and it's very unusual to see it used with PHP 7.2).

The fix is to re-enter the SMTP password and resave.  Hopefully this solves your problem!