Need a lot more information:
1. What exact Mac model do you have?
2. What exact Mac OS are you running?
3. When did this issue start?
4. Have you tried working with your Internet Service Provider regarding this issue?
5. Are you using Apple's Mail program? Seems like a number of folks are having recent various issues with it.
6. Can you access gmail from your browser? And if so, can you send and receive messages from there?