Glossary

Email, in plain English.

Clear definitions of the deliverability and list-monetisation terms you will run into, written for operators.

B

Blocklist: A DNSBL, a list of IPs or domains known for spam. If listed, many providers reject or filter your mail until you are delisted.
Bounce rate: The percentage of emails that could not be delivered. High bounce rates damage sender reputation.

CAN-SPAM: A US law setting rules for commercial email, including a valid sender address, a clear unsubscribe and no deceptive subject lines.
Catch-all address: A domain set to accept mail to any address. It makes verification uncertain because invalid addresses are not rejected.
Click rate: The percentage of recipients who clicked a link. A stronger signal of engagement and intent than opens.

Dedicated sending domain: A domain used only for sending campaigns, kept separate from your main website domain so sending activity does not risk your primary domain.
Delivery vs deliverability: Delivery means the receiving server accepted the email; deliverability means it actually reached the inbox. An email can be delivered yet still land in spam.
DKIM: DomainKeys Identified Mail, a cryptographic signature on your emails that proves they came from your domain and were not altered in transit.
DMARC: A DNS policy that tells receivers what to do with mail that fails SPF or DKIM, and sends you reports. It ties authentication together and guards against spoofing.
Domain reputation: The reputation tied to your sending domain. It follows the domain across providers and is rebuilt slowly, so it must be protected.
Double opt-in: Asking a new subscriber to confirm via email before they are added, which improves list quality compared with single opt-in.

Email deliverability: Whether your emails reach recipients' inboxes rather than being blocked or sent to spam. It depends on sender reputation, authentication, list quality and content.
Email verification: Checking whether an address is valid and safe to send to, by testing syntax, domain and mailbox, to remove dead and risky addresses before sending.
Email warm-up: Gradually increasing sending volume from a new domain or IP while generating positive engagement, so mailbox providers learn to trust it before full-volume campaigns.
ESP: Email Service Provider, a service that sends bulk email on your behalf and handles the sending infrastructure. Examples include Resend, Amazon SES and Mailgun.

Feedback loop: A service from a mailbox provider that tells senders when recipients mark mail as spam, so those addresses can be suppressed.

GDPR for email: EU and UK data rules requiring a lawful basis, often consent, to email people and handle their data, with the right to be removed.

Hard bounce: A permanent delivery failure, usually because the address does not exist. Hard-bounced addresses should be suppressed immediately.

Inbox placement rate: The percentage of delivered emails that reach the inbox rather than spam. A truer measure of success than delivery rate.
IP reputation: The reputation of the IP address mail is sent from. Shared IPs pool reputation across senders; dedicated IPs isolate it.

List hygiene: The ongoing practice of removing invalid, inactive and risky addresses so your list stays deliverable.
List-Unsubscribe: An email header that lets recipients unsubscribe directly from the inbox in one click. Providers increasingly require it for bulk senders.

Open rate: The percentage of delivered emails that were opened. A signal of subject-line strength, though less reliable as privacy features inflate it.

Reputation Guardian: Fortitude Send's system that monitors bounces and complaints per domain, provider and inbox, and automatically throttles or pauses sending before reputation is damaged.

Seed list: A set of test inboxes across providers used to check where a campaign lands, inbox, promotions or spam, before sending to the full list.
Send-time optimisation: Choosing the best moment to send to each recipient based on when they tend to engage, to lift opens and clicks.
Sender reputation: A score mailbox providers assign to a sender based on past behaviour. Strong reputation lands you in the inbox; poor reputation sends you to spam.
Soft bounce: A temporary delivery failure, such as a full mailbox or a server problem. Repeated soft bounces are treated like hard bounces.
Spam complaint rate: The percentage of recipients who mark your email as spam. Providers cap this very low, often around 0.1 to 0.3 percent, and exceeding it harms deliverability quickly.
Spam trap: An address used by providers and blocklists to catch senders with poor list hygiene. Hitting traps can get your domain blocklisted.
SPF: Sender Policy Framework, a DNS record listing which servers may send email for your domain. Receivers check it to confirm mail is authorised.
Subdomain sending: Sending from a subdomain such as mail.yourbrand.com, to isolate sending reputation from the root domain.
Suppression list: A list of addresses you must never email again, such as hard bounces, complainers and unsubscribes. Honouring it protects reputation and compliance.

We run the sending, protect the reputation and create the campaigns. You approve.