Selective use of Mandrill

You can switch Mandrill on for each mail sending module, Webforms, Ubercart, without switching Mandrill on for everything.

One big problem

When you switch the Mandrill module On in the Mandrill config page, the Mail system settings are wiped out. Before you use Mandrill, record all the settings in Mail system. Add the Mail system settings back after you switch Mandrill on.

My first test

My configuration for first tests:
Mail system and Mandrill are switched on.
Mandrill mail interface is set to On in Mandrill config page.
Mail system now shows the Site-wide default mail system interface class is MandrillMailSystem.
Switch the Site-wide default mail system interface class back to DefaultMailSystem.
Select New Setting.
Select Mailchimp >> Mandrill module in New Setting.
Save settings
Mandrill module class now appears in the Mail system settings list.
Select MandrillMailSystem as the class for Mandrill module class.
Save settings

You can now send a test email through Mandrill from the Mandrill module pages without anything else going through Mandrill.

You can then switch Mandrill on one at a time for each mail sending module, Webforms, Ubercart, whatever needs it, without switching Mandrill on for everything.

(This page started as a discussion in http://drupal.org/node/1817506)