When I send a newsletter, it is sent using a different email address than the one set on Newsletter/Settings.
You’re using an SMTP plugin
When you use an SMTP plugin it could be configured to force the sender email to the one set on that plugin. Check for this option (WP Mail SMTP has it).
If the SMTP plugin is not forcing the sender address but you configured the connetion to GMail, Google will set the sender address to the one used to setup the connection. If you use the Google Apps, you can reate a new mailbox and connect the SMTP plugin to GMail using that email.
You set a custom email on the newsletter you’re testing
Every newsletter can have a custom sender address, you can set it on the targeting page. Check if, for that newsletter, a custom address has been set.
You have the problem only with Automated generated newsletters
Check if the automated channel has a custom sender email set.
You’re the Newsletter/SMTP addon
Our addon does not change the sender email, but it could be forced by your SMTP provider: ask them.
You’re using a delivery addon
There are no reports about email forced by delivery provider connected with our addons.