1

I will send a mail in the evening or I will send a mail evening, Which one is correct? Actually what i am saying is, he can expect an email today evening from me.

So for today evening, I can say in the evening or just evening?

2

"In the evening" is correct. To be slightly more casual, you could also say "this evening" or "tonight"

Not the answer you're looking for? Browse other questions tagged or ask your own question.