Run these steps:
1. Write down you ISP mail server settings for SMTP & POP or IMAP.
2. Quit Mail
3. Go to the folder /Users/YourUserName/Library/Preferences/ and delete the file com.apple.mail.plist .
4. Launch Mail and first put in the server settings you wrote down.
5. Try to get Mail. If you have a server problem go to the Mail menu item "window" and select 'Connection Doctor' to see the errors you might be having.
To speed up Mail read the hint [HOWTO] a possible Fix for slow Mail.
Mac Pro Dual 2.8 Quad (2nd gen), 14G Ram, Two DVD-RW Drives, OS X 10.8.3
2006 Mac Book Pro 2.16 (first Gen) OS X 10.7.4
2TB Time Capsule, 2 TB
32G iPhone 4S Black, iPad (3rd Gen) 32G Black