I have lost portions of my mail app, how can I reinstall it?

Can you explain what happened? How come you lost portions of an application?! This does not make sense to me.

Do you mean some your mail messages or mailboxes are missing or some other kind of problems?

Please be as specific as possible in order that we can understand your situation and be in a position to give actionable advice.

Like luis said, walk through this step by step and clearly. If you don't know what;'s going on or don't know the right terminology -

say so and explain what you see.

But your question makes no sense.

Grant