R1:98c8e150b074

Prevent delivery of email to disabled objects

Summary: See T625. Facebook's REST-based MTA layer had a check for this so I
overlooked it in porting it out. We should not attempt to deliver email to
disabled users.

Test Plan:
Used MetaMTA console to send email to:

- No users: received "no To" exception.
- A disabled user: received "all To disabled" exception.
- A valid user: received email.
- A valid user and a disabled user: received email to valid user only.

(Note that you can't easily send to…