One sending domain for both password resets and newsletters is the most common deliverability mistake in small products, and it is invisible until the first campaign goes wrong.
Reputation is per domain, and per IP
Receivers build reputation from the domain in the From header, the domain in the return-path, and the source IP. If a marketing blast produces a spike in complaints, that reputation damage applies to the same stream that delivers password resets. Separating the streams means a bad campaign cannot lock users out of their accounts.
The operational difference
Transactional mail is triggered, low volume, and expected by the recipient within seconds. Marketing mail is batched, high volume, and can tolerate delay. They need different queue priorities, different retry policies, different suppression semantics, and usually different providers.
The suppression difference matters most: an unsubscribe from marketing must not stop a receipt the customer explicitly asked for. If both streams share one suppression list, either you break compliance or you break receipts.
A concrete split
Send transactional from mail.example.com and marketing from news.example.com. Publish separate SPF and DKIM for each. Give each its own DMARC policy — the transactional subdomain can go to p=reject quickly because it has few sources; the marketing one needs longer. Monitor them separately, because averaging them hides the one that is failing.
Volanea treats each sending domain as its own authentication and reputation unit, which makes this split a configuration choice rather than a migration.
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