Allow instances to force contact sender email

Certain email servers (most, all?) prohibit traffic
that appears to be routed from someone on the network

For instance, if all mail from an OTM site is going to
jane@company.com and jim ("jim@company.com") sends feedback
the message structure will look like:
From: jim@company.com
To: jane@company.com
....
But mail.company.com will *reject* the email since it is
actually originating from otm's servers and masquerading
as company.com
Merge pull request #86 from ahinz/topic/email

Allow instances to force contact sender email