One domain sending both “Your OTP is 482910” and “50% off this weekend” forces mailbox providers to average your reputation. When a promo campaign spikes complaints, your login emails can start landing in spam.
Recommended architecture
- notify.company.com — OTP, receipts, security alerts (transactional)
- news.company.com — newsletters and campaigns (marketing)
Each subdomain gets its own SPF/DKIM alignment and its own suppression list. Bounces on marketing should not block OTP delivery to unrelated users.
Consent and content rules
Marketing requires opt-in and an unsubscribe path in most jurisdictions. Transactional messages are tied to an existing relationship — but adding ads to a receipt still erodes trust and can trigger spam filters.
Volume patterns
Transactional volume tracks daily active users; marketing volume tracks campaign calendar. Peaks look different. Separating streams lets you set rate limits and warming schedules appropriately.
Platform choice
Some vendors sell cheap credits for raw SMTP only. Platforms like Mailmatic bundle logs, bounce handling, campaigns, and multi-domain management — useful when you run both lanes but want one dashboard and INR billing.