How to customize the SMTP message
Those messages are shown as the result of enabling logging in the SMTP module. It's only set once from the smtp_debugging variable. If it's still logging after disabling it, you to debug the issue as there's no one cause (e.g. caching, forcing the setting in settings.php/settings.local.php, code base hack, etc.) and is out of scope of Q&amp;A.

As for altering the "Further instructions have..." message, there are many ways to do this (e.g. form alter, translation files), but the simplest is probably utilizing the String Overrides module which will let you override any message wrapped in a t() function.