Why you should separate transactional and marketing email
Why transactional and marketing email belong on separate subdomains: how shared sender reputation lets a marketing campaign drag 2FA codes into spam, how to classify the gray-area emails that sit between the two, and what the legal rules add on top.
A password reset and a weekly newsletter have almost nothing in common: different audiences, expectations, legal requirements, and engagement patterns. Mailbox providers still judge a sending domain by how all of its mail behaves together, so sending both from the same domain ties their fates together. That shared reputation is how a marketing campaign can push transactional emails into spam, and putting each stream on its own subdomain is what keeps them apart.
The problem with one domain
Everything sent from yourapp.com shares the same sender reputation.
Mailbox providers like Gmail don't distinguish a password reset from a product announcement by
category; they see one domain and judge it as a whole.
Marketing email runs lower engagement and higher complaint rates than transactional email. Open rates of 20-25% are healthy for marketing, but for transactional anything below 50% signals a problem. On a shared domain, the marketing numbers drag down the overall reputation.
A 0.2% complaint rate on a promotional campaign is enough to make Gmail suspicious of the whole domain, which pushes order confirmations into Promotions and verification codes into spam. No alert fires when this happens, so the problem usually surfaces only after delivery has already degraded.
How subdomain separation works
The fix is to give each email type its own subdomain: mail.yourapp.com for transactional emails and news.yourapp.com (or marketing.yourapp.com) for campaigns.
Each subdomain then builds its own reputation, and Gmail treats mail.yourapp.com and news.yourapp.com as separate senders with separate
histories. A complaint spike on the marketing subdomain no longer touches the transactional
reputation, so password resets keep reaching the inbox regardless of how the marketing subdomain
is performing.
Setup means configuring SPF, DKIM, and DMARC for each subdomain independently. That sounds like double the work, but in practice it's a second set of DNS records. Lettr supports multiple sending domains, each with its own authentication and reputation.
A common setup splits the two streams like this:
mail.yourapp.comfor password resets, verification codes, receipts, and 2FA.news.yourapp.comfor newsletters, product updates, and feature announcements.
Once each subdomain is sending, register both in Google Postmaster Tools and watch their reputations diverge. The dashboards report per-domain spam rate, IP reputation, and auth pass rate.
The gray areas
Not every email lands cleanly in "transactional" or "marketing." A handful of common types sit on the line:
- Onboarding sequences. The welcome email is transactional; the day-three drip pitching unused features is marketing.
- Review requests. "How was your recent purchase?" is triggered by a user action but exists to drive engagement, so it counts as marketing.
- Activity digests. A weekly summary fires on a schedule, not a user action, which makes it marketing and means it needs an unsubscribe link.
- Renewal notices. "Your subscription expires in 3 days" is transactional; bolt on "here's 20% to come back" and the discount turns it into marketing.
- Shipping updates. "Your package shipped" is transactional until product recommendations under the tracking number push it into marketing.
When in doubt, treat it as marketing. The downside of routing a borderline email through marketing is slightly lower open rates. The downside of routing it through transactional and having it generate complaints is damage to the reputation of the emails users actually depend on.
The legal reinforcement
Transactional and marketing emails are governed by different legal rules, and mixing them creates compliance risk on top of inbox placement risk.
Under CAN-SPAM, transactional emails are largely exempt from opt-out requirements: a password reset needs no unsubscribe link. Marketing emails do require one, and the FTC has fined companies for noncompliance.
GDPR treats transactional email as necessary for contract performance: a purchase earns a receipt without separate consent. Marketing emails require explicit opt-in consent that recipients can withdraw at any time. Canada's CASL has similar carve-outs for transactional messages tied to an existing business relationship.
Gmail is particularly aggressive here. Its filters evaluate content signals, not internal categorization. A transactional email that looks promotional gets treated as promotional, regardless of what triggered the send.
FAQ
Why separate transactional and marketing email?
Should I use a subdomain or a completely separate domain?
Which emails count as transactional versus marketing?
Can one email be both transactional and marketing?
Bottom line
Separating the two streams is a structural decision, not a deliverability tactic. Any product that sends both password resets and newsletters benefits from putting them on different subdomains from the start, so a marketing campaign can never reach the reputation the transactional email depends on.
Lettr is built around separate sending domains, each with its own authentication and reputation, keeping transactional email and marketing campaigns apart by design. Create a free account to set transactional and marketing email apart.