Explicitly test for reply-to support before using the public reply-to address

Summary: We need to perform an explicit test for public reply support.
Previously, the existence of a valid result here was a sufficient implicit test
for public reply support, but it no longer is.
Test Plan: With an unmodified configuration, sent email. It generated with the
correct reply-to (me). Restored my original configuration and sent an email, it
generated with the correct (routed) reply-to.
Reviewed By: codeblock
Reviewers: codeblock
CC: aran, codeblock
Differential Revision: 626