The missing phpmailer module
There is a module for that: https://www.drupal.org/project/module_missing_message_fixer

And you can try drush or devel according to the doc here https://www.drupal.org/node/2487215

Use Drush

drush sql-query "DELETE FROM key_value WHERE collection='system.schema' AND name='module_name';"


When done, clear the site's caches (e.g. drush cr).
Also make sure that the CMI folder is cleansed of the broken uninstalled module. There may be some yml files left over and / or some system config. (this is all not the best way to do things as a FYI. However with Drupal 8 being so young, things don't go as planned always).

Use Devel

Enable Devel module and go to /devel/php. This will allow you to run some PHP code and you can execute the following:

$query = \Drupal::database()-&gt;delete('key_value')
    -&gt;condition('collection', 'system.schema')
    -&gt;condition('name', 'module_name')
    -&gt;execute();