Only messages less than 10MB in size seem to come through, does POPcon impose a size limit?

No, POPcon works fine with emails larger than 100MB but Exchange is normally set up to not accept Emails larger than a certain size. Unfortunately there are three different settings you need to check:

1. In the Exchange System Manager > Server > YOURSERVER > Protocols > SMTP > Properties of the Virtual Defaultserver for SMTP.

   

   
   

2. In the Exchange System Manager > Global Settings > Properties of Message Delivery > Defaults.

   

   
   

3. In the Active Directory entry of the individual user > Exchange General tab > Delivery Restrictions.