how to increase message size limit

[GabyR]

I'm using Plesk 10.2.0 and created there a service plan with mailbox size unlimited. Unfortunately an email addressed to a mailbox of that subscription bounced because "Message size exceeds fixed limit". So I need to increase the *message* size also. How can this be done in Plesk?

As a workaround I increased the system message size with postconf, but this should be solved on mailbox or subscription level.