A client of mine would like to confirm current customer's email addresses. Is there a tool you recommend prior to sending out an email campaign? Would a program like Constant Contact do this just as well?
A lot depends on the reason for confirming the address. If they're sending a mass email from a tool like Constant Contact or Campaign Monitor, bounced emails (hard bounces, where the address no longer exists) don't count against you in any way. Just send, and collect the bounces for removal or verification (which I just do manually, because they're usually a small group you can weed through for typos, etc.)
Confirming emails in advance is best done at sign-up time. Use a double opt-in, where they sign up, get an emailed link which they confirm as proof the email is valid, and that they're the ones who signed up, and hard bounces should drop to near zero.
What's your client's reason for confirming the email addresses? I'll bet there's a simple solution once we clearly identify their goal.
This client isn't completely virtual or doing internet marketing. Eventually he may get a website created for an opt in box. Right now he wants to concentrate on active customers and provide them with a newsletter about what's going on with the company and other products he may start offering.
I'd just send the emails, and deal with the exceptions as the occur. If they've got 1,000 email addresses, even a 5% bounce rate is only 50 emails to verify, and half of those will be obvious typos like .coom or .co which happens all the time.
Never burn time or money doing something that really doesn't need doing. There's zero downside to bounced emails, and it could be costly and time-consuming confirming emails which are probably 95% correct anyway.