What Is an SMTP Relay?
TL;DR
An SMTP relay is a mail server that accepts a message from an authenticated sender and forwards it toward the recipient mail server. Application mail and bulk mail go through a relay rather than connecting to each recipient server directly, because the relay owns the queue, the retry schedule and the reputation of the sending IP.
How is an SMTP relay different from a mailbox provider?
A mailbox provider stores mail for a person and gives them somewhere to read it. A relay stores nothing for long: it takes a message, holds it in a queue, negotiates delivery with the receiving server, retries transient failures and reports permanent ones. Sending and receiving are separate jobs, and a relay only does the first.
Which one you send through changes what receivers see. Mail relayed through a third party arrives from that provider IP addresses, which is why the relay has to be authorised in your SPF record and why its reputation becomes part of yours.
What do you need to configure for a relay?
Four things, and skipping any of them produces mail that leaves successfully and arrives badly. Authenticate to the relay with SMTP AUTH over TLS, add the relay to your SPF record with an include term, publish the DKIM public key at the selector the relay signs with, and confirm the relay sending IPs have matching PTR records.
Then check that the alignment survives. DMARC needs the domain that passed SPF or DKIM to match the From domain, and a relay that signs with its own domain by default will pass its own checks while failing yours.
Frequently asked questions
No, and the difference is authentication. An open relay forwards mail for anyone who connects, which is why open relays were abused into extinction and why their IP ranges are listed. A modern relay accepts mail only from senders who authenticate.