These utilities would simplistically assume that the input domain
was always a valid DNS name, which is true for the majority of
things, but our internal webhook, TSA and xfer queue names include
http URLs and would choke these utilities.
We now fall back to using the input "domain" if the MX resolution
attempt throws an error; the error is typically a failure to parse
rather than something like NXDOMAIN which is generally returned as an
empty result.