Jeff encountered a mail delivery issue updating the ticket; here is his response:
 

RFC 2743 Section 4.1 states that the second component will be treated as
a hostname regardless of whether or not a DNS lookup succeeds.

   "When a reference to a name of this type is resolved, the 'hostname'
   may (as an example implementation strategy) be canonicalized by
   attempting a DNS lookup and using the fully-qualified domain name
   which is returned, or by using the 'hostname' as provided if the DNS
   lookup fails.  The canonicalization operation also maps the host's
   name into lower-case characters."

In the case of a name which begins with a leading underscore the DNS
lookup is guaranteed to fail.  I will call out two items in the above a
paragraph:

 1. Canonicalization by attempting a DNS lookup is optional.

 2. If the optional DNS lookup fails, the 'hostname' will be used
    as provided.

What I have pointed out in this ticket is that a getaddrinfo() query
will always fail for a name beginning with a leading underscore and that
the failure might take a long time.  Therefore, there is no benefit to
issuing the query and the canonicalization step described in Section 4.1
should be skipped.