I'm using unix authentication. My username in unix is "jschewe". My default domain is "@domain1.com". I have 2 email addresses: "jschewe@domain1.com" and "jon.schewe@domain2.com".

I created a test project that sends emails on any trigger to the developers.

I created a commit as "jon.schewe@domain2.com" and pushed my changes and manually triggered the build. This resulted in the following:
Email was triggered for: Always
Sending email for trigger: Always
Not sending mail to unregistered user jschewe@domain1.com
An attempt to send an e-mail to empty list of recipients, ignored.
The changelog for this build points to the Jenkins user account that I use to login that has the email address jschewe@domain1.com associated with it. This is not expected. I expected this to point to the user "jon.schewe@domain2.com".

I made a commit as "jschewe@domain1.com" and pushed my changes and manually triggered the build. This resulted in the following:
Email was triggered for: Always
Sending email for trigger: Always
Not sending mail to unregistered user jschewe@domain1.com
An attempt to send an e-mail to empty list of recipients, ignored.
The changelog for this build points to the Jenkins user account that I use to login. This is what I expect. However I would have expected the email to go out.

What is unclear to me is what exactly is a registered user when using the unix security realm?

How does the unix security realm map to the email addresses in git commits?

This may be related to #43178.