In Holland the use of English language is quite common (I was born there and visit frequently). The biggest Dutch newspaper (De Telegraaf) has just started a website called
www.gametoday.nl , this website is totally targeted at Dutch visitors (website language is also Dutch). They also will start soon a website called
www.newz.nl (now still redirected, also targeted at Dutch visitors). Another sample is
www.insidegamer.nl I know the guys involved and they recently sold this site to a Dutch magazine company, again targetted at Dutch visitors and language is Dutch. So basically an English name and Dutch (.nl) extension does not hurt at all and that makes it extra valuable IMO.
Mail.nl is a good catch, IMO it is worth high $$$$, basically if it was mine I wouldn't sell for less than $10,000 as I know in Holland there are several potential interested parties for this name. For example KPN, that is the biggest mail, internet and telephone company, they could be interested (maybe you can even contact them) and they also have no problem buying up English names as they just bought
www.mine.tv for a online tv website (interestingly mine.nl is owned by a Dutch bank), again mine.tv: English name, targetted at Dutch visitors, language is Dutch). Do not rush to sell this one, take your time and it will pay off IMO. Good luck! John.