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What is the difference between transactional email and marketing email, and why do most Node.js applications use separate providers or sending domains for each? What is an unsubscribe mechanism, and what specific requirements does it need to meet for compliance with regulations like CAN-SPAM? What is the difference between sending SMS via a provider's REST API (like Twilio) versus using SMTP-based email in terms of a Node.js integration? How would you test that a Node.js application correctly sends emails without actually sending real emails during automated tests? How would you design a multi-channel notification system in Node.js that can send the same logical notification via email, SMS, or push depending on user preference? How would you implement rate limiting or throttling for outgoing notifications to avoid overwhelming users or violating a provider's sending limits? How would you handle email delivery webhooks (bounces, complaints, opens) sent from a provider like SendGrid back to a Node.js application? How would you implement idempotent notification sending to avoid sending a duplicate email if a background job is retried? How would you design a notification preference center allowing users to control which types of notifications they receive via which channels?
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What are SPF, DKIM, and DMARC, and why do they matter for a Node.js application's transactional emails actually reaching users' inboxes?

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SPF (Sender Policy Framework) publishes a DNS record listing which mail servers are authorized to send email on behalf of a domain, letting receiving servers verify a message wasn't spoofed. DKIM (DomainKeys Identified Mail) cryptographically signs outgoing messages, letting receivers verify the message wasn't altered in transit and genuinely originated from the claimed domain. DMARC builds on both, publishing a policy telling receiving servers what to do (quarantine, reject) with messages that fail SPF or DKIM checks -- without properly configured SPF/DKIM/DMARC records, legitimate transactional emails are significantly more likely to be marked as spam or rejected outright.
; Example SPF DNS TXT record
v=spf1 include:sendgrid.net ~all

; Example DMARC DNS TXT record
v=DMARC1; p=quarantine; rua=mailto:dmarc-reports@example.com
Real-world example A company's password-reset emails were landing in users' spam folders at an alarmingly high rate until they properly configured SPF and DKIM records for their sending domain through their email provider's setup process, dramatically improving inbox placement.

Common follow-ups: How does DKIM signing actually work cryptographically to verify a message wasn't tampered with?;What's the difference between DMARC's 'none', 'quarantine', and 'reject' policy modes?

Security;HTTP & HTTPS Modules
All 15 Email & Notifications questions