We're having issues sending emails in processes: I am getting this alert in

We're having issues sending emails in processes: I am getting this alert in the log:
"Sender address rejected: User unknown in relay recipient table"
and this in Appian:
There is a problem with task “Send E-Mail” in the process “Update Password Reminder”. Failed to send email
Problem: An error occurred in executing an Activity Class
Our server is on a local machine (not cloud).
Can anyone suggest a solution?
NB:
1. The to address takes a user object, the user is active in the system, though we have tried non-system users & direct email addresses
2. There are no ACPs being handled at the node
3. The node runs as the designer...

OriginalPostID-79150

OriginalPostID-79150