Question : Exchange 2003 - Just started receiving 5.5.0 error with one site only.

What can i do proactively to help the receiving site with this error??
Is there some special patches needed??
Why Did it just start happening this past friday, and with only 1 site??

Answer : Exchange 2003 - Just started receiving 5.5.0 error with one site only.

Ok this is normally something that effects webmail but your bestbet is to start with the logs

In your Windows\System32\logfiles folder you should see a directory for SMTP..
Find the log that has the date of the error, open the log with notepad and do an edit/find search for "500 " or "5.5.0 "
You want to find the command that was issued, so it should be on the same line or the one above it just prior to the 5.5.0 error.
You may have to open IIS or in ESM's Protocols/SMTP branch and augment the details for the logging you are collecting.
You basically want to add all logging options using w3c format.

This should show you whats going out.  there is the possibility the max length for the syntax has somehow been altered.  It should be 1000 chracters by default.  I suspect you may have to check the regestry for the setting but I'm afraid  don't know the key as I haven't had to go that far with this issue before

Hope this points you in the right direction.