It'd never occur to me to send an email in reponse to a request for a lyric. I think do so would feel a rather unfriendly way to answer, though I can see some people mightn't see it that way. Most of the time anyway surely people don't put the email address in do they?

As for people not coming back again after posting a request, if they are guests I tend to assume they've quite likely come back with a new name as members.

The fun with requests for lyrics is that this can and should (in my opinion)lead into an exploration of aspects of the song in question, variants, discussion of what it's about, pointers to other songs which seem relevant. This isn't just a reference book or a slot machine. If I were to use an email it's probably be just to say that the lyrics had been posted on such and such a day, maybe if I'd come across the request after some delay when hunting through the archive.