Activation emails not received?

I get a lot of messages from new users who do not receive their activation emails. It seems to happen to allmost 5% of all new users.

There seems to be no problem with the domain being on a blacklist or something like that so I am a bit stuck with it.

I also fear the users who send me their support calls do not get my response either. If so, please contact me here or on the Facebook page and we will work it out.