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).
Which torsocks version? Yours is probably newer than mine. It seems to be a problem with torsocks 2.3.0 and only with dig. And indeed there is nothing wrong at the network level because I was able to do an MX lookup over tor using a different method than torsocks. I'm also able to use other apps with torsocks, just not dig all of the sudden.
It could be the exit node that you're using perhaps? Maybe you could try specifying a different exit node and trying again. Also check exit node policies to make sure DNS is allowed, although as your problem only seems to be with MX records then that might not be the cause.