In order to uninstall MassMailer, 


STEP 1:


Log in to your Salesforce org where the MassMailer app has been installed, go to the browser URL and replace the URL right after "....force.com/" replace the remaining URL with "/apex/Cloudrop__MassMailerUninstaller" and load the uninstaller page.


LIGHTNING:


These pictures show the lightning version.







CLASSIC:


These pictures show the classic version.






STEP 2:


Click on the 'Perform MassMailer Pre-Uninstall Actions'.



STEP 3:


Once the process in STEP 2 is over, follow the instructions below:


LIGHTNING:


Go to Salesforce Setup -> Quick find 'installed packages' -> click on 'Installed packages -> click on Uninstall link -> Follow the instructions to uninstall the MassMailer app.





CLASSIC:


Go to Salesforce Setup -> Quick find 'installed packages' -> click on 'Installed packages -> click on Uninstall link -> Follow the instructions to uninstall the MassMailer app.