This could have several causes, most likely that would be:
You are using a so called free mail account provided by hormaul, gmail etc. Mail accounts which can be quickly applied are default blocked by us, you wil have to use mail accounts which are provided by your Internet Provider or which belong to your domainname.
Registration can be disabled by us, if so then you will have to wait until we enable it again.
There is a bug on the website, you received an error or you did not receive the activation mail (but you did get an confirmation on this site) then follow the instruction in this FAQ