E-mail alias forward to Hotmail returns: Relaying denied. Proper authentication required

Hi,
On my web site, I have created a few alias e-mails addresses, which I am
forwarding to users.

The idea is to forward mail from an address on my domain (e.g.
myname@mydomain.xzx) on to another address such as myname@mywork.zxs

It works fine for most aliases.

But two of my users have Hotmail accounts.

When I send an e-mail til them via their aliasname@mydomain.xzx tien I get
this error mail:

The original message was received at Wed, 24 Jan 2007 11:41:29 +0100
from server3.i123.dk [193.31.15.18]

----- The following addresses had permanent fatal errors -----

(reason: 550 5.7.1 ... Relaying denied. Proper
authentication required.)

----- Transcript of session follows -----
.... while talking to mail01.i123.dk.:
>>> DATA
<<< 550 5.7.1 ... Relaying denied. Proper
authentication required.
550 5.1.1 ... User unknown
<<< 503 5.0.0 Need RCPT (recipient)

I'm sure that it worked a couple of months ago (November 2006), but now it
fails.

Any ideas?

Regards,
Lars