(stable) Fix bad "$this" references in "Must Encrypt" mail after MailEngine changes

Summary: See PHI1038. I missed these when pulling the code out.

Test Plan: Sent "Must encrypt" mail, verified it made it through the queue in one piece.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20029