Hi Christina,
This looks like you have probably have conflicting jar files (javax.mail jar file, jakarta.activation-2.0.1.jar, javax.mail-1.6.2.jar, activation-1.1.1.jar, and commons-email-1.5.jar.EmailTemplate) in your userlib exposing the same classes, but those classes are not compatible.
Have you upgraded to Email Connector from Email Template for example, and not cleaned out your application's userlib?
I would suggest going into the userlib folder and removing all duplicates you can find, keeping just the versions with the highest revision numbers in the filenames. Also, remove any .jar that are from old modules (javax.mail-1.6.2.jar, activation-1.1.1.jar, and commons-email-1.5.jar.EmailTemplate) that are no longer in use.
Hope it Helps!
Thanks & Regards,
Manikandan K