Mail server in Mac OS Server 5.1 (El Capitan)

Good morning.

A question about Mail Server!

I want to set NO limits for single email size (with attachments).

Default value seems to be max 10 MB...

For my needs it is not enough

Now when somebody send a big e-mail to one account on the server, receives this message:

SMTP error: 552 5.3.4 Message size exceeds fixed limit

This message is repeated many times.

Also, how can I stop it?

Can you help me?

Many thanks and best regards!