Microsoft's instant messaging network is rejecting users who have signed up using a .Mac email address.

Macworld has received reports which indicate that existing users of MSN (now Windows Live) have been knocked off the network when they try to sign in if they have originally used a .Mac address to sign up for the service.

In a moment's hubris, Microsoft insists users change their email address before using the service.

"Because of a recent system update, you must change the email address that you use to sign in to Windows Live Messenger. Until you change your email address, you won’t be able to use Windows Live Messenger," an error message explains.

Microsoft explains the changes were required as a security measure because Office Live Communications Server 2005 SP1 enables messaging between the Windows Live Messenger Service and corporate IM systems.

Microsoft has put together a support page for affected users.