While researching a problem with message size limits at a customer site the other day, I stumbled across a very interesting situation with a setting of "unlimited" for the organization-wide message size limits.

To configure organization-wide limits, you can use the Set-TransportConfig cmdlet to set the MaxReceiveSize and MaxSendSize, which controls the maximum message size that users can receive and send, respectively.

In my case, my customer wasn't ready to set any size limits (still working out policy and user communication issues).. so we set them to "unlimited":

Set-TransportConfig -MaxSendSize:unlimited -MaxReceiveSize:unlimited

So, that was all fine and dandy - except that we were still unable to send a message larger than 5 MB through OWA and through the clients. 

After doing a little more internal research - stumbled across an interesting throught.  Apparently, the term "unlimited" is misleading.  As it was written in the notes of a PSS case I found, "unlimited" means that the user can send an unlimited amount of the 5 MB default size limit, not that the size limit itself is unlimited.

Setting the MaxReceiveSize and MaxSendSize to 150 MB fixed our problems, and enabled us to send large attachments via e-mail.