Is the word thank in Thank you! a verb? If not, what part of speech is it then? If it is a verb, is it in the imperative mood? I'm asking because I've seen someone write
Do thank you!
which sounds wrong to me1. If it were an imperative verb, the above would be grammatically correct. That's why I started wondering what form it is morphologically.
1 Is it actually wrong? I understand one could say: I do thank you., but is Do thank you! valid? (I suppose it's meant to emphasize thank you)