If you are getting an error something like the following, you should contact Salesforce and enable the standard object 'Enhanced Letterheads' in Salesforce.
You can refer to this article published by Salesforce on the same.
Review the errors on this page.
- Cloudrop.MassMailerLeadConversionTrigger: execution of AfterUpdate caused by: line 481, column 9: Dependent class is invalid and needs recompilation: Class Cloudrop.MassMailerLeadConversionTriggerHandler : Dependent class is invalid and needs recompilation: Class Cloudrop.MassMailerBatchToQueue : Dependent class is invalid and needs recompilation: Class Cloudrop.MassMailerEmailAlert : Dependent class is invalid and needs recompilation: Class Cloudrop.MassMailerUtil : Invalid type: EnhancedLetterhead
Or if you are getting the following error while editing the Lead object.