Email deliverability glossary

Plain definitions of the terms that come up when setting up a sending domain or running permission-based campaigns. Where a guide goes into more depth, the definition links to it.

The product

DanixSoft: The software company that builds and operates OutreachPro.
OutreachPro: OutreachPro is a web application, built and operated by DanixSoft, that sends personalised email campaigns and follow-ups to opted-in contacts from the user's own Google Workspace or Microsoft 365 mailbox. How it works.

Authentication

BIMI: Brand Indicators for Message Identification: a DNS record that lets supporting mailbox providers show your logo next to your mail. It requires DMARC at an enforcing policy (quarantine or reject), and Gmail additionally requires a verified mark certificate.
DKIM: DomainKeys Identified Mail: a cryptographic signature added to each message and verified with a public key in the sender’s DNS. It shows the message was authorised by the signing domain and not altered, and it survives forwarding. SPF, DKIM and DMARC compared.
DKIM selector: The label that tells a receiver where to find a DKIM key: the key lives at selector._domainkey.yourdomain. Google Workspace commonly uses “google”; Microsoft 365 uses “selector1” and “selector2”.
DMARC: Domain-based Message Authentication, Reporting and Conformance: a DNS record stating what receivers should do with mail that fails authentication (none, quarantine or reject) and where to send reports. DMARC alignment explained.
DMARC alignment: The DMARC requirement that the domain authenticated by SPF or DKIM matches the domain in the visible From address. Relaxed alignment accepts subdomains of the same organisational domain; strict requires an exact match.
MX record: The DNS record naming the servers that receive mail for a domain. A sending domain with no MX record cannot receive replies or bounces, which receivers treat as a warning sign.
Reverse DNS (PTR): A record mapping a sending server’s IP address back to a hostname. Gmail requires it for all senders. When you send from Google Workspace or Microsoft 365, the provider’s servers already have it.
SPF: Sender Policy Framework: a DNS TXT record listing the servers allowed to send mail for a domain, checked against the envelope sender. Evaluating it may not take more than ten DNS lookups. The 10-lookup limit.

Reputation and delivery

Bulk sender: In Google’s and Microsoft’s rules, a domain that sends around 5,000 or more messages a day to their consumer mailboxes. Bulk senders must meet stricter authentication and unsubscribe requirements. What bulk senders must do.
Hard bounce: A permanent delivery failure, most often because the address does not exist (SMTP status 5.1.1). The address should never be mailed again. Hard and soft bounces.
Inbox placement: Whether a delivered message lands in the inbox, a tab, or the spam folder. The receiving provider decides it; a sender can influence it but cannot measure it exactly or guarantee it.
Sender reputation: How mailbox providers judge a sending domain and its servers, built from authentication, complaint rates, bounce rates, sending patterns and how recipients engage with the mail.
Soft bounce: A temporary delivery failure such as a full mailbox or a deferral (a 4.x.x status). The message is worth retrying, and the address should not be suppressed.
Spam complaint rate: The share of delivered messages that recipients mark as spam. Google requires senders to stay below 0.3% as reported in Postmaster Tools, and recommends below 0.1%.

Sending

Consent record: Evidence of how and when a person agreed to receive email: the source (such as a signup form), what they were told, and the date. OutreachPro requires one for every imported address. What counts as consent.
Double opt-in: A subscription confirmed twice: once by submitting a form, and again by clicking a link in a confirmation email. It proves the address is real and belongs to the person who signed up.
Email warmup: Increasing a new or quiet mailbox’s sending volume gradually over weeks, so receivers see a steady pattern rather than a sudden burst. In OutreachPro it is a ramp on your own campaign mail, not an exchange of fake messages. What warmup can and cannot do.
Mailbox rotation: Spreading one campaign’s messages across several connected mailboxes, each within its own limits, so no single mailbox carries an unusual volume.
One-click unsubscribe: The List-Unsubscribe and List-Unsubscribe-Post headers (RFC 8058) that let a mail client unsubscribe someone with a single click. Gmail and Yahoo require it from bulk senders of marketing mail.
Open tracking: Recording an open when a tiny image in the message is loaded. It over-counts, because privacy features and image proxies load images automatically, so reply rate is the more reliable signal. Why open rate misleads.
Sending limit: A cap on how much a mailbox may send in a day or an hour, set by the provider (Google Workspace allows 2,000 messages a day per user) and, well below that, by you. Provider sending limits.
Sending window: The days and hours, in a chosen timezone, during which a campaign is allowed to send.
Sequence: A series of messages sent to the same person over time, where each follow-up is sent only if a condition holds, such as no reply yet.
Suppression list: Addresses that must never be mailed again: people who unsubscribed or complained, and addresses that hard-bounced. In OutreachPro it applies across the whole account.
Tracking domain: The domain used in open-pixel and click-redirect links. Using a subdomain of your own instead of the tool’s shared one keeps links consistent with your brand.