$ glossary
The words that decide whether an email arrives.
Every term here is something a receiving mail server checks, or something you will be asked about when mail stops arriving and nobody can say why. Definitions first, then how each one actually breaks. If you want the sequence rather than the definitions, how email authentication works puts these pages in reading order.
Authentication
The three checks a receiving server runs before it reads a word of your message, and the fourth question that decides whether they add up to anything.
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SPF
SPF is a DNS record listing which servers may send mail for your domain. It authenticates the envelope sender, which is not the address the recipient sees.
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DKIM
DKIM adds a cryptographic signature to outgoing mail so a receiver can verify the message was authorised by the signing domain and has not been altered.
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DMARC
DMARC ties SPF and DKIM to the address a recipient actually sees, tells receivers what to do when neither matches, and reports back on what was sent as you.
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DMARC alignment
Alignment asks whether the domain SPF or DKIM authenticated matches the domain in the From header. A message can pass both checks and still fail this one.
read it →
Infrastructure
What the machine doing the sending looks like from the outside, which receivers weight heavily and rarely tell you about.
Hygiene
What to do with the addresses that reject you, and why doing nothing is the fastest way to lose a sending reputation.
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Hard bounce
A hard bounce is a permanent rejection — the address does not exist. A soft bounce is temporary, like a full mailbox, and is worth retrying for a while.
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Suppression list
A suppression list is the set of addresses a sender will not mail again — hard bounces, spam complaints and unsubscribes — enforced before a message is sent.
read it →
Sending
The shape of the thing being sent, the ways an application hands it over, and the conditions large receivers now attach to accepting it.
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Transactional email
Transactional email is mail an application sends to one person because that person did something — a receipt, a password reset, an alert, a confirmation.
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Bulk sender requirements
Since February 2024, senders of over 5,000 messages a day to Gmail or Yahoo must authenticate with SPF, DKIM and DMARC, align, and keep complaints low.
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SMTP relay
An SMTP relay is a server that accepts authenticated mail from your application and delivers it onward, so your app never talks to recipient servers.
read it →
Or check your own headers first.
Every definition here describes a check you can run against your own mail today. If they are failing, Pharos fixes them as a side effect of onboarding a domain.
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