I have set up Freepbx on a Raspberry and it has been running fine. I using Mailjet to process the emails that are sent out as notifications to me. Recently, I have received notices from Mailjet that an “unknown sender” is trying to send messages and identifies the domain of the sender as "freepbx.MYDOMAIN.com (with MYDOMAIN being my valid domain name). I need to check the email addresses that freepbx uses as sender to send me those notifications. Where are all the places, files, etc. that could be used to set those addresses?
If you purchased the (relatively cheap) System Admin module, then from the FreePBX dashboard, it’s Admin > System Admin > Email Setup. I prefer this module to keep everything in one place for defining things such as this. Rather than manually editing files through SSH and the like. 
I would by the module, but this is an experimental setup on a cheap Raspberry PI5 (which works surprisingly well and stable), and it seems the SysAdmin module is not available for it. So, I think I have go the SSH route. which is fine. I just need to know where I have to look …
The FreePBX mail engine is likely Postfix. If you search online for the standard docs for this commonly-used mail engine, then you can see how and where it’s configured.
Thanks. That’s what I did. I updated a few files, and am hoping it is resolved now.