If a mail is sent from a POP3 user (or alias), the previously saved passphrase in the User GUI will not be automatically applied and the e-mail will not be encrypted.
The POP3 user sends the mail via SMTP with the original alias - example@domain.com, but actually has the alias example+domain.com@pop3proxy.reddoxx assigned to him in the User Administration.
Because of this discrepancy, the mail will not be automatically encrypted with MailSealer Light.
Add the original alias (example@domain.com) in the Admin Webinterface -> Administration -> Users and Groups -> E-Mail Addresses and assign the respective user to the new alias.