How to Smtp relays

Published October 4, 2024 · Updated August 11, 2026 · SMTP & Deliverability

SMTP relays are the quiet infrastructure that carries your email from where it is created to where it is read. Understanding how relays work - and choosing a good one - is central to reliable delivery. Here is a clear overview of SMTP relays.

How SMTP relays work

When you send, your message passes through one or more relays that accept and forward it, applying authentication and a sending IP along the way. The reputation of that relaying IP heavily influences whether the message reaches the inbox.

Choosing the right relay

Dedicated IP - reputation you control.
Real volume without hidden caps.
Automatic SPF, DKIM, DMARC.
Easy integration on port 587.

The decision that matters: shared relays share reputation; a dedicated relay isolates yours. For email that matters, dedicated wins.

The hop-by-hop journey of a relayed email

A message rarely takes a single hop. It typically moves from your app or mail client to your relay's submission server (port 587), which authenticates you, then queries DNS for the recipient domain's MX records, then connects to the recipient's mail server and hands the message off — each hop adding its own headers that record the path, which is exactly what lets a receiving server (and you, if you inspect the headers) trace where a message actually came from.

When a relay chain breaks

Most delivery failures trace to one of three points: the initial authentication (wrong credentials or an app password issue), a DNS lookup failure on the recipient's MX records (rare, but happens with misconfigured domains), or the receiving server rejecting the message outright because SPF, DKIM, or DMARC failed. Reading the bounce message's exact wording almost always tells you which of the three it was.
A quick reputation self-check
Search your own sending IP or domain against a free blacklist-lookup tool periodically — catching a listing before it affects a real send is far less costly than discovering it after a campaign underperforms with no obvious explanation.
Note: If you manage multiple domains, check each one's relay reputation separately — a clean record on one domain does not guarantee the same for another.
A relay's reputation is built over time through consistent behavior — a single well-authenticated send does not undo months of inconsistent sending, and vice versa.
Related reading: Email Domain Management · Email Warmup Guide: How to Build Sender Reputation on a New SMTP Server · test your email deliverability

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Frequently asked questions

What are SMTP relays?

Servers that accept and forward your outgoing email toward recipients, applying authentication and IP reputation that decide inbox placement.

How do I choose an SMTP relay?

Prioritise a dedicated IP, real volume, automatic authentication, and simple port-587 integration.