a.) write back DIAF, because sure, they probably don't know what it means, but it is not professional.
b.) check urbandictionary for more acronyms.
c.) smallcap your response. Or use the word typevore even ironically when referring to their typing.
You can be so sarcastic you wonder if it would have been politer to simply say DIAF and be done with it, though. Passive-aggressive is awesome.
If you want me, I will be framing emails to maximize my intent to be bitchy, and in pursuit of this goal, my grammar must be flawless.