Are there any free services out there, that will test the validity of my mailing list? I know most providers want you to be under 5% bounce rate, and I want to be sure the emails the client provided me with are corrent.
The foolproof way is to make the registration process (if that is what it is) a two stage event. So at the end of the first stage you send out an email with an embedded link which completes the second stage.
I do understand thats the best way when accumulating a list, but this is a list that already exists that my customer wants to send an e-blast out to. I want a way to check these email addresses to be sure they are valid.
Are you sending the emails directly yourself? If so, that's your first mistake... it's not just about staying off a list, getting whitelisted is hugely important these days too. Work with a provider with a history, stable addresses and good relationships.
As far as bounces, there's no magical way to forecast a good address although some people will try to connect to each address up front. Instead why not consider triming the length of your list? Someone who signed up 2 years ago and hasn't been emailed since is really not a part of your database at this point. Focus on the more recent members and start removing the bad names up front to avoid this issue down the road.