Is there any way to verify the number of subscribers to my mailing list ( as selling point for advertisers ) ? (not the US TV host)
Maker of WIP amongst other things.

So they know the number you quote is in fact valid? I've never been asked that question. As long as the results (e.g. clicks) seem legit, I think most advertisers will trust your word.

I'm with Marc on this. Most advertisers don't actually care about the exposure size, they prefer results. In fact every advertiser I know would rather get 50 hits from being shown to 100 people than 500 hits from being shown to 1,000,000 people.

I think the companies that care about reach are more likely to be brand advertisers, eg Coke.