You need to register e-mail addresses that shall be used as e-mail senders, e.g. in the Email print destination.
Steps
In D365 FSCM go to System administration > Setup > dox42 email senders.
Select the New button.
Enter the desired values in the Email and Name text fields.

dox42 e-mail senders page
Select Save.
Result
You have registered an e-mail address.