Multi SMTP builds on top of the SMTP Authentication Support module, which in turn requires the phpmailer library.
If phpmailer is missing, Multi SMTP fails silently. This should display a prominent warning to admins to let them know that mailings will not work without it.
Comments
Comment #1
BrockBoland commentedFix has been committed.
There is also a fix in the smtp module dev version, but they haven't released it yet: #541942: Move PHPMailer out of the module folder, check for class already defined