Agent WranglersDeliverability Recommended
Separate transactional and marketing mail
Send transactional mail and marketing mail from different subdomains, and from different IP addresses when you control them.
How to do it
Use, for example, mail.example.com for receipts and password resets, and news.example.com for newsletters. Each subdomain gets its own DKIM key and builds its own reputation. Then a newsletter that annoys people never delays a password reset. Give each subdomain its own SPF, bounce address and DKIM selector, and keep the visible From aligned with it so DMARC passes.
How to check
Read the headers of one receipt and one newsletter. The DKIM d= domains differ, and so does the envelope sender.
For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers email playbook: https://agentwranglers.se/playbook/email/deliverability/separate-streams.