Configure SMTP “From” Address

I run all of our transactional and marketing messages through a custom-built SMTP server, and I now need every single message we send to show a new “from” address. That address has not yet been added or verified on the server, so the first step will be setting it up properly, completing whatever SPF, DKIM, and verification steps are required, then updating the server configuration to make it the default sender. Once the address is live and trusted, I want the change enforced globally—no edge cases where an older identity slips through. After you update the SMTP settings, I’ll need a quick proof: send a few sample messages my way and show the headers so I can see the new address, correct DKIM signature, and a pass on SPF. If you have experience with Postfix, Exim, or any other common MTA and you’re comfortable troubleshooting DNS records and mail-flow issues, this should be a straightforward task.