Advanced Email Verifier will check every e-mail address from a given mailing list and determine whether e-mails are valid. AEV is easy to use, but powerful and reliable utility to verify and clean up your mailing list.
List hygiene plays a role in the delivery race. It is important to maintain a clean mailing list and remove bounced/undeliverable emails because a lot of ISP mail servers have been known to block a sender's email domain for repeated sending messages to non-existing email addresses.
Avoid sending newsletters to bad addresses. If you ultimately decide to use a commercial service to manage your list and send your newsletters, you will usually pay either by the number of sent messages or by the size of your list.
If you handle the newsletter mailing yourself, then it becomes a time issue. In other case, you will have to deal with bounced messages. There is no need to disturb your clients and friends anymore to check if their e-mail addresses are still valid.