My general feeling is that, if an average client machine is making a MX query, most likely it has been taken over by a spambot sending out spam. I have been using Microsoft Outlook and other MUAs (mail user agents) with Deadwood for over a year while its MX processing was broken and never saw a problem; a MUA does not perform MX lookups to send mail, but instead connects to the SMTP server, which does the actual MX lookups.
That in mind, in today’s snapshot of Deadwood, I have explicitly disabled MX lookups; if an MX lookup is made, the query is rejected and the IP is logged as being a possible spam zombie. There is a parameter which I have documented that will enable MX lookups again.
The fact of the matter is this: Using Deadwood with a mail hub making a large number of MX requests is completely untested. If one wants to use Deadwood in this manner, please subscribe to the MaraDNS mailing list and express the intention to help beta-test Deadwood for this use case which I do not have.
In the meantime, by disabling and logging MX lookups in Deadwood, we can make networks a little more secure and resistant to spam zombies commandeering machines on our networks.