Most email problems are gradual. This one is abrupt: a marketing campaign goes badly, and a week later customers cannot receive password reset emails. The two events look unconnected and are the same event.
The distinction that matters
Transactional email is triggered by something a specific person just did. They reset a password, made a purchase, changed an account setting. Volume follows your product's activity rather than a schedule, engagement is high because the recipient is expecting it, and complaint rates are near zero.
Marketing email goes to many people, at a time you chose, about something they did not just request. Engagement is far lower, and complaints are inevitable at some rate no matter how good the campaign is.
The test is not whether the content is promotional. It is whether this specific recipient's own action caused this specific message.
Why the reputations must not be shared
Mailbox providers assess reputation per sending domain. They do not know or care that you consider some of your mail important and some of it promotional — they see aggregate behaviour on a domain.
So when a campaign draws complaints, the reputation of the sending domain falls. If that domain also sends your password resets and receipts, those now arrive slower, land in spam, or are rejected. Your customers cannot log in, and the support tickets that follow will not mention email at all.
Separating the streams contains that failure. A campaign can go badly without taking your account recovery with it.
How to separate them
Subdomains, for customer marketing
A common and effective arrangement:
yourcompany.com— corporate mail and, if you wish, transactional.mail.yourcompany.com— marketing campaigns to existing customers.notify.yourcompany.com— transactional, if you want it fully isolated.
Each subdomain needs its own DKIM key and builds its own reputation, while inheriting some trust from the parent domain. That inheritance is why subdomains warm up faster than fresh registrations.
Separate domains, for cold outreach
Cold outreach draws complaints at a materially higher rate than marketing to your own customers, so subdomain separation is not enough. Use entirely separate domains with no connection to your primary one, as covered in our cold email guide.
Separate infrastructure, not just addresses
Domain separation handles reputation. There is a second reason to separate the sending path itself: transactional and marketing mail have opposite requirements.
Transactional mail must go immediately. A password reset arriving twenty minutes late is a broken product. Marketing mail should be paced deliberately, with randomised gaps between messages, precisely so it does not look like a burst.
A system tuned correctly for one is tuned wrongly for the other. This is why Email.biz.pk is not the right tool for transactional email and says so plainly on our SendGrid comparison — an API relay is the correct choice for application mail, and deliberate pacing is the correct choice for campaigns.
Do not mix them in one message
The most common version of this mistake is adding promotional content to a transactional message — product recommendations under a receipt, an offer inside a shipping notice.
It is tempting, because transactional mail has excellent open rates. Two problems follow. Legally, mixing commercial content into a service message can bring the whole message under commercial email rules, including the unsubscribe requirement. Practically, it trains recipients to treat your transactional mail as marketing, and some will mark a receipt as spam — which is precisely the outcome the separation exists to prevent.