Today my mail server (patched 8.13.6) went and stopped delivering mail
for some of my toy domains.
This mail server is MX for those domains (receiving only though) and
delivers with catchall entries in virtusertable:

@toydomain.com some@internal.domain

Error is something like:

Running /var/spool/mqueue/l3NEpam09907 (sequence 1 of 5)
<some@toydomain.com>... toydomain.com: Name server timeout
<some@toydomain.com>... Transient parse error -- message queued for
future delivery

while something like
echo test | sendmail -v -d8.20 test@toydomain.com

shows a series of lookups for toydomain.com (AAAA A then MX), all
failing since that server does use an internal (private) dns server with
a configuration which doesn't get any data for that specific domain
(because not configured internally and no recursion to public name
servers allowed).
However this didn't ever stop sendmail from using the address
some@internal.domain specified in virtusertable, until now.

I couldn't find anything changed in any sendmail or bind configuration.
Is this normal behaviour, lookup for the MX of the destination domain
(and fail if no A or MX records found) even when it will be changed by
virtusertable anyway ?
In other words, did it work by chance before but shouldn't have had
(because the lookups before virtusertable expansions are normal and
records must be present) or should it still work now (and how can I find
what is wrong ?