Don't blow up when email has no References header
What does this MR do?
Don't blow up when the
References header isn't present and the
To header doesn't match; raise a normal exception instead.
Are there points in the code the reviewer needs to double check?
Not sure. Should I be using that fixture?
Why was this MR needed?
This is a part of fixing https://gitlab.com/gitlab-org/gitlab-ee/issues/2247.