Definition
An SMTP relay is a mail server or service that accepts outgoing email and forwards (relays) it to the destination's mail servers. It sits between your application and the recipients, and is how most transactional and bulk email actually gets delivered.
Relays handle volume, connection management and delivery logic, and reputable relay services provide authentication, reputation management and monitoring to keep mail flowing and trusted.
For developers and marketers, choosing a relay matters because its reputation is shared — your mail's deliverability depends on the relay being trustworthy. Use a relay with strong authentication, modern TLS and good volume handling, and treat its credentials as sensitive.
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Related Glossary Terms
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Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
BCC in Email
BCC (blind carbon copy) hides the addresses of recipients added to it so other recipients cannot see them. Learn how BCC is used in email settings, privacy and coordination.
Best Time to Send
The best time to send is the moment an email is most likely to be opened and read, varying by audience, industry and behaviour. Learn how to find and use your optimal send times.
Email Blocklist
A blocklist is a database of senders, IPs or domains flagged for spam, which providers consult to filter mail. Learn how a blocklist placement hurts delivery and how to recover.
Email Bounce Back (Deferred Delivery)
An email bounce back — also called a deferred delivery — is when a receiving server temporarily holds a message and retries rather than accepting or rejecting it. Learn how bounced-back mail is handled and resolved.
Bounce Escalation
Bounce escalation is the point at which a repeated soft bounce or deferral becomes a hard bounce, ending attempts to an address. Learn how bounce escalation works and why handling it protects reputation.
Frequently Asked Questions
It is a mail server or service that accepts outgoing email and forwards it to destination mail servers, handling the delivery of most transactional and bulk email.
A relay manages volume, connections and delivery, and reputable services provide authentication, reputation and monitoring so your mail is delivered reliably.
Yes. Because the relay's IP reputation is shared across senders using it, a trustworthy relay with strong practices protects and improves your email delivery.