MailTester.com allows you to verify if an e-mail address is valid or not.
This can help you keep your contacts database up to date.
MailTester performs a series of tests (DNS lookups, SMTP checks) on an e-mail address to determine its validity.
Unlike other products MailTester will also tell you if verification is not possible (e.g. for catch-all mail servers).
The site is free for individual checks, but there is also a commercial batch e-mail verification tool to check entire lists of e-mail addresses (e.g. in an Excel sheet or a database).