Hello Nergetic, I use Infacta Group Mail since forever, and it works like a charm.
It has a built-in add-on which is capable of cheking email validity, with two levels of validation: DNS check and SMTP check.
Both services will check for email validity with different level of validation (and different time needed to complete the test).
I think that you should validate them in two hours or so (by using both methods for a more precise validation).
Hope this helps.