What’s the danger of mixing transactional and marketing sends?

Mixing transactional and marketing mail on the same domain or IP creates contamination risk:

Marketing generates complaints at higher rates than transactional mail. Some recipients mark promotions as spam while expecting receipts.

Contaminated reputation affects all mail from that sending identity. If marketing tanks your domain reputation, transactional receipts may land in spam too.

Different engagement patterns mean aggregated metrics misrepresent each stream. Providers may filter based on blended signals that do not reflect transactional quality.

Operational risk where marketing problems delay or prevent critical communications like password resets, order confirmations, or account alerts.

Best practice separates streams using dedicated subdomains for each mail type.

Mixing streams is like shipping fine china with industrial equipment. When the heavy cargo shifts, it damages the delicate items you actually need to protect.