I have run into the max message size limit alot lately especially in sbs 2011 installs. most people need the limit increased to at least 20 meg. you can follow this documentation to do so. this can also be done with the gui however the exchange powershell makes these changes much faster.
Size limits for email messages are controlled in a variety of places in Exchange. Settings are placed globally at the transport level, on each individual send or receive connector, at each individual mailbox, and controlled at individual web applications such as EWS and OWA . Your business needs should determine if you need to change these settings and at which location(s). In order to accomplish this successfully, you must know that more restrictive settings override less restrictive settings as you flow up the chain from individual mailbox settings to SMTP connectors and finally to global transport settings.
Global Transport Settings
Get-TransportConfig displays global transport settings for all Hub and Edge Transport servers in the Exchange Organization. By default, there is a 10MB size limit on send and receive. This will override all other less restrictive message size limit settings set elsewhere.
NOTE: Exchange considers message submission to the store driver as a “send” operation. Therefore, if you are attempting to receive an email message on an SBS server from the internet that is greater than the MaxSendSize of the global transport configuration, delivery will fail.
Get-TransportConfig | ft MaxSendSize, MaxReceiveSize
Set-TransportConfig –MaxSendSize 20MB –MaxReceiveSize 20MB
Receive Connector Settings
Each receive connector is created with a default 10MB message size limit. A default install of SBS will contain a default receive connector (created during Exchange setup, do not remove this), an internet facing receive connector, and a receive connector for fax receipts and Sharepoint email notifications. The Windows SBS Internet Receive connector is the only one that you should ever need to adjust settings for. NOTE: If the global receive size is more restrictive, it will override this setting
Get-ReceiveConnector | ft name, MaxMessageSize
Set-ReceiveConnector “Windows SBS Internet Receive Servername” –MaxMessageSize 20MB
Send Connector Settings
Each send connector is created with a default 10MB message size limit. In SBS , a single send connector is created for email destined for mail hosts outside of the site. If you need to adjust the size of messages flowing out of the SBS server, then you would change it here. **If the global send size is more restrictive, it will override this setting**
Get-SendConnector | ft name, MaxMessageSize
Set-SendConnector “Windows SBS Internet Send Servername” –MaxMessageSize 20MB