To do an MX lookup over Tor, this command has worked for me for years:
$ torsocks dig @"$dns_server" -t mx -q "$email_domain" +noclass +nocomments +nostats +short +tcp +nosearch
In the past week or so it just hangs. My first thought was the DNS server I chose (8.8.8.8) started blocking tor. But in fact it does not matter what DNS server is queried. The whole Tor network is apparently blocking tor users from doing MX lookups.
Also notable that dig hangs forever. It does not timeout despite a default timeout interval of 5 seconds (according to the man page).
Exit nodes are temporary unless you deliberately pin them for a particular connection, which I have not done for the DNS servers. The problem manifests without exception for weeks now, so it could not be attributed to a bad exit node. The tor microdescriptor db tracks the perms of every node, so I don’t think it would create a circuit for disallowed traffic. There could be an inconsistency between the microdesc and reality, but it would have to be a replicated inconsistency for every connection attempted with torsocks and yet not replicated on any connection made using the torsocks alternative (which works).