Email notification error

Hi,

I tried to find a post on this but couldn't, hope someone can help me. A guestbook I installed (v2.3.3) is working fine, but when I get a notification email the person's email who signed the guestbook appears like this:

'name@.MISSING-HOST-NAME.'

so the first part of their email is correct but I don't get the hotmail.com or whatever. Then when I open the message, in the from field it says:

'From: name@localhost, MISSING_MAILBOX_TERMINATOR@.SYNTAX-ERROR.'

Could anyone tell me what that means and is there any way to fix this?

Cheers,

Grant