Nico,
I'm not familiar with those firms, but you could also check RingLead and Demand...
The big issue is that your lead database, like everyone's, is losing value and quality every day. I've seen estimates that a database will decay 2% or more each month...or 25%+/yr.
So while a lead verification service may help identify possible emails that are still valid, the real question is whether or not that lead is still remotely interested in you. There's no point in finding out a 7 year old lead is still active when they've forgotten your firm or are in a completely unrelated role now.
To clean this up, I suggest:
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Dedupe.
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Leads over 2 years old need to be checked for activity of any type.
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Leads under 2 years old also need a check for activity within past 6-12 months.
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Any email that is hard bounced needs to be flagged and marked as such.
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Find and remove Spam Trap emails.
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Consider removing email addresses with lots of soft bounces.
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Now that you've removed or blocked much of the poor leads, then you can consider a wake from the dead campaign to get people to click on a link and on your site.
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At this point an email verification service can be a bit more helpful.
See my chapter on this:
http://www.slideshare.net/jdavidhill/marketo-email-reputation-management