Email infrastructure in plain English

The percentage of sent emails that fail delivery and return a bounce notification. Hard bounces are permanent (invalid address); soft bounces are temporary (mailbox full). Target hard bounce below 0.5%.

The percentage of recipients who clicked a link in your email. Since Apple's Mail Privacy Protection broke open rate in 2021, CTR is the cleanest engagement signal email gives you, because a click is a deliberate human action.

A sending IP used by one sender only, so its reputation is entirely yours. Gives full control and isolation from other senders, but needs consistent volume (roughly tens of thousands a month) to stay warm. Low-volume senders usually do better on an established shared pool. Requires warmup.

A cryptographic signature your domain attaches to outbound email so receiving servers can verify the message actually came from you and was not tampered with along the way. One of the three pillars of email authentication, alongside SPF and DMARC.

The policy layer that tells receiving mail servers what to do when SPF or DKIM fails on mail claiming to be from your domain. Also the reporting standard that surfaces who is trying to impersonate you. Sits on top of SPF and DKIM.

The p= field in your DMARC DNS record. Tells receivers what to do with mail that fails both SPF and DKIM alignment. Three values: none (just report), quarantine (deliver to spam), reject (refuse outright). The percent flag and subdomain policy let you ramp safely.

A subscription method where a new subscriber must confirm by clicking a link in a confirmation email before joining your list. Blocks typos, bots, and spam traps at the door and proves consent. The trade-off: 20-30% of sign-ups never confirm.

The DNS records that prove a message came from the domain in its From line. SPF lists allowed sending servers, DKIM signs each message, DMARC ties both to the visible From and sets the failure policy. Required by Gmail, Yahoo, and Outlook for bulk senders since 2024.

A published list of sending IPs, domains, or URIs that mailbox providers consult to reject inbound mail. Spamhaus, Barracuda, SORBS, plus private internal blocklists at every major mailbox provider. Blacklist and blocklist are interchangeable terms.

The art and science of getting email into the inbox instead of the spam folder. Covers sender authentication, IP and domain reputation, list hygiene, ISP signals, and the operational practices that keep all of it healthy over time.

Gradually increasing sending volume from a new domain so mailbox providers build a positive reputation before you hit full volume. Distinct from IP warmup, which builds reputation for the sending IP.

The percentage of recipients who opened your email, measured by a tracking pixel. Apple's MPP (2021) now pre-fetches that pixel whether or not the user opened the message, which makes open rate far less reliable than it used to be.

The deliberate limitation of email sending rate, either self-imposed (for warmup, reputation recovery) or imposed by the receiving mailbox provider (via 421 responses). Different from pausing (a hard stop); throttling slows the flow without stopping it.

An HTTP POST your sending platform makes to your URL every time something happens to a message: delivered, bounced, opened, clicked, complained. The push side of event tracking, faster and cheaper than polling an API.

A managed platform that handles the operational side of sending email: authenticated infrastructure, deliverability tooling, bounce processing, analytics, and a direct line to mailbox providers. Different from an MTA (the raw engine) and from a mailbox provider (the receiving inbox).

The programmatic channel mailbox providers use to notify senders when a recipient marks their mail as spam. Yahoo CFL, Microsoft JMRP, Comcast SmartFBL, and others. Enrolling and processing FBL reports is table stakes for legitimate senders.

A permanent email delivery failure returned by the receiving mail server. Invalid address, dead domain, or rejected sender. Suppress immediately on first occurrence.

The share of accepted email that actually lands in the inbox rather than spam. Different from delivery rate, which only confirms the server accepted the message. Measured with seed lists and provider panel data like Google Postmaster, since mailbox providers don't report folder placement to senders.

The gradual ramp of sending volume from a new IP so mailbox providers can build a reputation profile for it over time. Skip it and your first big campaign lands in spam. Do it right and the IP earns trust that pays back for years.

The ongoing practice of removing addresses that hurt deliverability: hard bounces, repeat soft bounces, complainers, the chronically unengaged, and likely spam traps. A smaller engaged list out-delivers a big stale one. Run bounces/complaints continuously and engagement pruning quarterly.

The receiving end of email: Gmail, Outlook, Yahoo Mail, Apple iCloud. They decide whether your message lands in the inbox, the promotions tab, or the spam folder. Different from an ESP (the sender-side platform) and MTA (the raw engine).

The server software that actually moves email over SMTP. Postfix, Exim, and Sendmail are the major ones. What matters for deliverability is less which MTA you run and more how it is operated: warmup, reputation control, and retry discipline.

The dashboards Gmail and Microsoft (SNDS) publish so sending domains can see their own reputation, spam rate, and authentication status from the receiving end. The inbox's own view of your sending.

The envelope MAIL FROM address where bounces go, distinct from the visible From header. It is what SPF authenticates against, which is why DMARC alignment can fail even when SPF passes.

The trust score mailbox providers calculate for every sending domain and IP, from bounce rate, complaint rate, engagement, authentication, and consistency. The single biggest input into whether email lands in the inbox or the spam folder.

The POSIX system call for transmitting a message on a socket. Covers scatter-gather I/O and ancillary control data including SCM_RIGHTS file-descriptor passing. Also: the inspiration for our brand name, since at the lowest layer every email reduces to one of these.

The plain-text, store-and-forward protocol that moves email between servers. Standardized in 1982 and still underneath every email you send. Read the actual HELO/MAIL FROM/RCPT TO conversation and the three ports (25, 587, 465) explained.

A server that accepts outbound email from your application and forwards it to recipient mail servers on the open internet. The mechanical workhorse layer behind every email API.

A temporary email delivery failure returned by the receiving mail server. Mailbox full, server overloaded, or brief throttle. Retry with exponential backoff before suppressing.

An email address operated by mailbox providers or anti-spam organisations specifically to catch senders with poor list hygiene. Pristine traps were never used for real email; hitting one signals bought, scraped, or appended data and can drop your reputation a full tier in 24 hours.

A DNS record that lists which mail servers are allowed to send email on your domain's behalf. Receiving servers check the connecting IP against the list to spot forged senders. The oldest of the three authentication standards, still essential.

Automated email sent in direct response to a user action — password resets, OTPs, order confirmations, shipping notifications. One-to-one, expected by the recipient, time-sensitive. The opposite of a marketing campaign.

Transactional email is triggered by a user action and expected (OTP, receipt, password reset). Promotional email markets to a list. They differ in consent, timing, and sending, and mixing their reputations is a common, costly mistake.