The Program downloads large messages, but does not deliver them to users.

MPC receives mail from the external POP3 server then sends it through the internal SMTP receive connector. MPC does not have settings which would limit the size of messages which are sent through SMTP Connector Exchange

SMTP Receive Connector (which it is chosen in MPC settings) has two parameters which can limit the size of the messages received by it:

MaxMessageSize - This parameter specifies the maximum size of the message. Default value: 10MB.

And
ConnectionTimeout - This parameter specifies the maximum period of time which a connection can remain opened, even if active data transmission is being carried out through the connection. The default value for the connector for reception, which is set in the transport server-concentrator, is 10 minutes. The default value for a connector for reception, which is set in the boundary transport server, is 5 minutes. The specified values should be entered as time intervals: dd.hh:mm:ss, where dd — days, hh — hours, mm — minutes, and ss — seconds. The value specified in the parameter ConnectionTimeout should be more than the value specified in parameter ConnectionInactivityTimeout. The admissible range of values of any of these parameters is from 00:00:01 to 1.00:00:00.

You can determine the speed of the connection of the connector with the SMTP receive connector in the log of the diagnostic utility of MAPILab POP3 Connector. Using the speed and the desirable maximum message size, it is possible to find the required timeout value.

Changing receive connector parameters is possible using the command :
Set-ReceiveConnector-Identity "Connector name"-ConnectionTimeout 00:30:00 – MaxMessageSize 30Mb