Don't send newsletters from your transactional domain

Use a separate subdomain for marketing email than for the mail your product depends on. This is the cheapest deliverability decision available, and it is nearly impossible to make retroactively.

What reputation actually attaches to

Receivers score senders, and the domain is one of the strongest signals. Complaints, spam-folder placements and bounces accumulate against the domain that sent them.

Your two kinds of mail behave completely differently:

• Transactional — password resets, receipts, alerts. Expected, opened, almost never reported as spam.
• Marketing — newsletters, announcements. Sent to people who half-remember signing up. Some will report it as spam. That is normal and unavoidable, even when everything is done correctly.

Share one domain and those complaint rates average together. A campaign that draws an ordinary number of complaints drags down the reputation that your password resets depend on.

The failure is not dramatic. It shows up as reset emails arriving in spam, and support tickets from people who cannot get back into their accounts.

The setup

Two subdomains of the domain you already own:

mail.yourcompany.com     password resets, receipts, alerts
news.yourcompany.com     newsletters, announcements

Each gets its own DKIM key and its own SPF record. Reputation is then tracked separately, and a bad campaign is contained to the subdomain that sent it.

Send from subdomains rather than the bare domain, too. Keeping yourcompany.com out of bulk sending protects it, which matters because it is also the domain your invoices and your staff's mail come from.

Why later is expensive

Moving marketing mail to a new subdomain after a year means starting from zero reputation for that stream, exactly when you have volume. New domains get throttled, so the first campaigns from the new subdomain deliver worse than the ones you were trying to fix.

Splitting on day one costs one extra DNS setup.

The part people miss

Separate domains only help if the mail is genuinely separated. Sending a "product update" newsletter from your transactional subdomain because it is "not really marketing" puts you back where you started.

The test is not what you call it. It is whether a recipient might reasonably report it as spam. If yes, it belongs on the marketing subdomain.