Is it - send your response to Mary and I or to Mary and me?

Answer 1

"Send your response to Mary and me" is correct.

It is always incorrect to say "to Mary and I" in any context.

The easiest way to keep track is to substitute "we" or "us" for "Mary and I/me." "Mary and I" is equivalent to "we"; "Mary and me" is equivalent to "us."

• We went to the store ⇒ Mary and I went to the store.
• Send it to us ⇒ Send it to Mary and me.

That will guide you naturally to the correct selection, except in the case of predicate nominatives. For example, "It is we" is grammatically correct, so you would substitute "It is Mary and I," but most people think either of those sounds odd.