SMTP Problem: data: 354 Enter message, ending with "." on a line by itself

[eluser]Unknown[/eluser]
Hello,

I am using codeigniters email library via smtp to send PDF files to members of the website. It works very well until it comes to certain email addresses. In the debug report, it comes out with:

Code:

to: 550 Unrouteable address
data: 354 Enter message, ending with "." on a line by itself

The email does not send to the address. Same thing happens whether it's html/plain text/no attachment/sendmail/phpmail.

It works on my Gmail account, hotmail account and a couple others I tested. So I know the script works as it should be, but I have no idea what this error means, where the origin is or how to resolve it 9as far as I know, it is most likely a strict server rule for spam? maybe?). It was difficult trying to find info on the error with google and what I could find made absolutely no sense to me.

If anyone could shed some light or point me in the right direction that would be awesome.

Cheers

[eluser]Narf[/eluser]
Maybe the (non-?) receivers happen to use MS Exchange 2000?
http://support.microsoft.com/kb/813050