Is there any way to verify the number of subscribers to my mailing list ( as selling point for advertisers ) ?
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.