If you are problems connecting to a specific website or problems receiving/sending email you may need to Flush your computers DNS.

On a Windows system, open your CMD prompt (MS-Dos Window) and type in the following command:
ipconfig /flushdns

On a Mac OS X Leopard system, open a command terminat and type in the following command:
dscacheutil -flushcache

On a Mac OS X 10.5.1 or below system, open a command terminat and type in the following command:
lookupd -flushcache