Invitation problem

Hello.

I sent invitations to some of my friend............ but they didn't take it after more than a day. How can it be? What is it the problem? I can see on the invitation tab the sent letters (10), but my friends said they didn't take anything (they saw in the spam folder too).

Thanks
Necromancer

Thanks the info. There were problem with hotmail and gmail (these are the international mail service providers), and two hungarian: citromail.hu/freemail.hu. I really surprised only on the gmail. Other thing. Normally how long time will it take from the sending to the delivering? Another idea. Can I send a new invitation for my friends, but the system write out: The person already invited. Free this up or how long time will it take to send a new one? And you can check the registration status of the email address (yes or not). Because in this situation I can send a new mail if my friend(s) don't take the invitation. Now I can't do it......
Again.. Thanks for the answer. I hope i could help you a little bit with the list of the email providers.

Regards
Necromancer