Email, written down
Notes on deliverability, transactional and marketing email, and what we’ve learned building email infrastructure at scale.
Why your emails go to spam (and how to fix it)
A diagnostic guide to why emails go to spam: how spam filters actually decide, where to find the evidence (message headers, Google Postmaster Tools, Microsoft SNDS, blocklist checks), and the five causes behind most spam placement, each with the signs that identify it and the fix.
How to send bulk email the right way
How to send bulk email that reaches the inbox: the sending limits that rule out Gmail and Outlook mailboxes, the bulk sender requirements Gmail, Yahoo, and Microsoft now enforce, the legal rules in the US, EU, and Canada, and a six-step workflow from permission to monitoring.
Email API: what it is and how to choose one
What an email API is and how it works: the request and response shape, how it differs from an SMTP relay and from mailbox APIs like the Gmail API, what an SMTP API means, the criteria that matter when choosing one, and the free tiers available in 2026.
Best transactional email services in 2026, compared
A comparison of nine transactional email services (Lettr, Postmark, SendGrid, Mailgun, Amazon SES, Resend, MailerSend, Brevo, SMTP2GO): current free tiers, pricing at 50,000 and 100,000 emails a month, deliverability track records, and which service fits which use case.
SMTP relay: what it is and how to set one up
What an SMTP relay is, how it differs from a mailbox SMTP server, how relay authentication and the TLS ports (465, 587, 2525) work, a step-by-step setup you can copy, and how a relay compares to IMAP and to a REST email API.
Hard bounce vs. soft bounce: what email bounces mean and how to handle them
What an email bounce is, the difference between a hard bounce (a permanent failure such as an address that does not exist) and a soft bounce (a temporary one such as a full mailbox), the common causes of each, why a bounce rate above roughly 2% damages sender reputation, and how to handle both kinds correctly.
Mailer Daemon and Mail Delivery Subsystem errors explained (and how to fix them)
What a Mailer Daemon or Mail Delivery Subsystem bounce-back means, the common reasons an email fails to deliver, how to read the SMTP status code in the message, step-by-step fixes for each cause, Gmail and Yahoo specifics, and what bounces that arrive for mail you never sent are telling you.
Email attachment size limits by provider (and how to send large files)
The attachment size limits for Gmail, Outlook, Yahoo, and iCloud Mail, why the real ceiling is lower than the number suggests because of encoding overhead, and the practical ways to send a file that is too big to attach, including large video.
Onboarding email best practices (with examples and templates)
A practical guide to onboarding email sequences: what an onboarding email is, how to time and structure the welcome-to-activation flow, the best practices that matter (subject lines, personalization, a single CTA, the metrics to track), teardown examples, copy-paste templates, and how to send the sequence programmatically.
Email deliverability checklist
A complete email deliverability checklist covering authentication (SPF, DKIM, DMARC), list hygiene, content and spam triggers, sender reputation and IP warmup, and the monitoring metrics to watch, with a printable summary at the end.
Email signature size and dimensions
The recommended pixel dimensions, file size, and format for an email signature, how signatures render in Gmail, Outlook, and Apple Mail, and the common mistakes that make them break on mobile or load slowly.
Introducing marketing emails in Lettr
Marketing email now lives inside Lettr. The new module adds Audiences for managing who you send to and Campaigns for designing and sending bulk email, all in the same account and on the same domains you already use for transactional sending.
Managing Lettr from your AI assistant: the MCP integration
How Lettr's MCP integration lets an AI assistant call the Lettr API in plain language: what MCP is, what the tools cover, the difference between the remote and local servers, how to set up each one, and which tasks suit a chat interface versus the dashboard.
The hidden cost of DIY transactional email
The real cost of running your own transactional email stack: deliverability drift, no delivery logs, on-call time spent on bounces and blocklists, and template changes that require a deploy. What DIY actually costs beyond infrastructure, and what a managed alternative replaces.
How to warm up a new sending domain
How to build sending reputation on a new domain by ramping volume gradually: who needs a warm-up, which recipients and content to send first, two sample ramp schedules, the bounce and complaint signals to watch, and the stop signals that mean pause immediately.
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.
SMTP vs. REST API: how to choose and when to switch
How SMTP and REST API email transports differ in practice (statefulness, serverless fit, templates, idempotency, error handling), when each is the right choice, and a step-by-step plan for migrating from SMTP to a REST API.
SPF, DKIM, and DMARC explained for developers
What SPF, DKIM, and DMARC each do, how they fit together to stop email spoofing, and how to read the DNS records and authentication headers to tell whether they're working.
The journey of an email: from API call to inbox
A step-by-step walkthrough of what happens to an email between your API call and the recipient's inbox: validation, message assembly, DKIM signing, DNS routing, the SMTP handshake, authentication, content filtering, and placement.
How transactional email actually works
What separates transactional email from marketing email, why inbox providers and regulators treat them differently, and how to keep these messages reliable in production.