Thanks Oz, Art, FlakeZ, Galactus. It wasnt like this before. What's strange is my boss who uses the same mail server dont have this problem. He can send and receive without getting the error.

I can send and receive messages within our network. Like if i send to same email domain, no problem. Pero pag sa iba na ako nagpapadala...nirereject ng server yung recipient but here's another thing, strangely, I can receive messages from outside pero hindi ako masend ng reply sa kanila.

Puzzling.