My Exchange 2007 does not accept emails larger than 10MB. What can I do?

This is due to the default limit of 10MB in the Exchange 2007 receive connector. You can find this setting under Server Configuration, Hub Transport, Receive Connectors, Properties of the Default Receive Connector:

Exchange 2007 Receive Connectors

The Maximum message size is fixed to 10240KB (10MB) by default on the General tab of these properties Change that to 102400 (100MB) in order not to allow longer emails to come through. You can always limit individual user’s allowed maximum email sizes.

You can also use the Exchange Management Shell: View current limits with the Exchange Management Shell command

get-receiveconnector | select identity,maxmessagesize

and change them with

set-receiveconnector "Connector Name" -maxmessagesize 50MB

set-receiveconnector "*default*" -maxmessagesize 50MB

to 50MB for example.