* stable-3.0:
OutgoingEmail: Sanity check recipients of multipart and plaintext
OutgoingEmail: Log when email is sent as multipart and plaintext
OutgoingEmail: Log the reason that email is not being sent
OutgoingEmail#send: Check first if email sending is enabled
Upgrade jackson-core to 2.10.0
Change-Id: I51a03d657f0631b1913be10ff8cb7dafca42232b