Do you get this error in Outlook and Exchange 2007.
Finding the fix took me a bit of time, so here is how to fix it.
I cannot remember where I found this, but kudos to whoever posted it.
Start adsiedit and locate
Configuration => Configuration => Services => Microsoft Exchange => First Organisation => Global Settings => Message Delivery
Right click the Message Delivery container, and choose Properties. In the Attribute list, locate DelivContLength and SubmissionContLength.
I suspect both values are set to something like 10240. If the values are present and set, double click both attributes, and then click the Clear button followed by the OK button. At this point the value should be <Not set>.
In my case, an unlimited size was not desireable, so do a little math. The 10240 roughly equals a 10MB attachment. You can set something like 100000 for 100MB or 25000 for a 25MB attachment.
Now, either allow for a replication cycle to go through, or force a replication. Then restart the Exchange Services. Perform a test to see if the actions performed yielded the desired results
The screenshot shows the first of the two. In my virtual that was behaving like your environment, both of these had a 10240 in them.