I've been asked this question, though never via work email, and I've answered (if I could). Of course this was mostly in Holland, where I could just refer people to the nearest coffee shop.
If I knew them personally, I'd tell them of my favourite place (not for the smokes because I haven't used in years, but for the staff and the awesome music) and the one with the best reputation when it comes to quality.
In London, I'd tell you to go to a certain area (very touristy and plenty to see even if you're not after drugs), not because I've bought anything there, but because you only have to walk around and people will whisper to you: "Dope? Pills?"
I'm 100% certain the police know about this practice in that area (they probably know most, if not all, of the dealers anyway). I assume people know that drugs are illegal here in the UK when they visit, so I don't feel I should warn them about it. Not my business.
If the question was asked through my work email
, I'd ignore it and give the other information.
I tend to end up in jobs where the boss would be the person to ask that question, though.