I have a customer with the same problem, I'll try to explain in other words...
Normally, the email client (outlook, thunderbird, etc.) will connect to iredmail smtp server with username+password for each user of domain, so for example email@example.com will connect to the smpt with firstname.lastname@example.org + infopassword and email@example.com will connect to the smpt with firstname.lastname@example.org + marketingpassword, and so on.
With exchange server, the problem is that the exchange server can use only one smtp account server for all the mailboxes, for example email@example.com with relative password.
If a client send from firstname.lastname@example.org no problem occurs, but if another client try to send with another account like email@example.com the iredmail server can't send the email.
I hope it's clear now :-)