my mail won't let me send E-mails becasue it says i'm offline, but i can receive E-mails fine.

hey, my mail on my macbook Pro isn't working properly. It basicly will reseive E-mails fine but when i want to reply to them it won't let me becasue it says i'm offline. so i try and turn it to online, but it still won't let me send them. if any one could help that would be brilliant. i hate having to open up hotmail every time to reply to an email.