Is phpmailer on another Ubuntu instance? If so the error log is going to be located at /var/log/mail.err or the specific event may be logged in /var/log/mail.log
The important log will be the one on the MiaB instance itself.
MiaB is on a linode server, PHPList is on another server.
phplist SMTPDebug is set and it shows this:
Connection: opening to box.mybox.com:587, timeout=5, options=array ()
Connection: Failed to connect to server. Error number 2. "Error notice: stream_socket_client(): unable to connect to box.mybox.com:587 (Network is unreachable)
SMTP ERROR: Failed to connect to server: Network is unreachable (101)
SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
MiaB /var/log/mail.log shows nothing when I try to send a mail from phplist.
The log entry seems to suggest that the PHPList server cannot access the MiaB server on port 587, and the MiaB server showing nothing seems to support this.
From the PHPList server, do you get something similar to below:
$ curl box.mybox.com:587
220 box.mybox.com ESMTP Hi, I'm a Mail-in-a-Box (Ubuntu/Postfix; see https://mailinabox.email/)
221 2.7.0 Error: I can break rules, too. Goodbye.
Has the provider of the other server blocked mail ports by chance? It looks to me as though you are unable to communicate out on port 587 from the error message that you posted. Maybe you can reach out to support and ask them if mail ports are blocked and to unblock if so.