Thanks for al the very useful answers.

I was able to change my BTC address before, but it just not works anymore.
My mail server logs do not show any attempt to receive a mail from them except from the support team.
It is 100% sure no problem with spamfiltering or mailsettings.

It seems confirmed that the problem has to do with scam, not spam.

The advice to stay away from HashFlare seems to be the best after all.

The only good news is that the (luckily) small amount of money I lost with them was not needed.

So my advice to all :
HashFlare is a proven SCAM ! Stay away from it.