SMTP Relay for SaaS Apps: What to Use and Why
What an SMTP relay is, when a SaaS app should use one, how it differs from an email API, the providers to consider, and the DNS and production tradeoffs.
Overview
An SMTP relay is a third-party service your app connects to over SMTP to actually send email, instead of running your own mail server. You hand it the message; it handles delivery, IP reputation, and the hard parts of getting to the inbox. For a SaaS app, a relay is the sane default — running your own outbound mail server means owning IP reputation, blacklist fights, and maintenance you almost never want.
What it is and when to use one
Use a relay whenever your app or an off-the-shelf tool sends email and you don't want to run a mail server (which is almost always). It's especially the right call when you're integrating software that only speaks SMTP, or when you want portability — point the SMTP credentials at a different provider and you've switched senders with a config change, no code rewrite. That portability is the main reason to choose a relay (SMTP) over an email API.
Most major providers offer both an SMTP relay and an API over the same backend: SendGrid, Mailgun, Brevo, Amazon SES, and self-hostable useSend. So "relay vs API" is usually a choice of interface with the same provider, not a different deliverability outcome.
- A relay sends your mail so you don't run a mail server
- Best when integrating SMTP-only software, or for portability
- SMTP = swap providers with a config change, no rewrite
- Most providers offer both relay and API on one backend
Setup and production tradeoffs
Setup: get SMTP credentials (host, port, username, password/API key) from your provider, store them as secrets, and prefer a submission port with TLS (587, or 465 implicit TLS) — never send credentials in the clear. Then do the same authentication work as any sender: SPF, DKIM, DMARC on your domain, plus bounce and complaint handling.
Tradeoffs vs an API: SMTP is universal and portable but chattier and gives coarser error feedback, and reaching advanced features (idempotency, rich events) is harder. For high-volume product email from your own backend, an API is often nicer; for portability and third-party tools, the relay wins. Either way, keep transactional separate from marketing and monitor deliverability.
- Use TLS submission ports (587/465); store creds as secrets
- Still need SPF/DKIM/DMARC + bounce/complaint handling
- SMTP: portable and universal, but coarser feedback than an API
- Keep transactional and marketing streams separate
Common Mistakes
- Sending product-critical mail and marketing from the same domain/reputation.
- Skipping idempotency, so retries create duplicate sends.
- Ignoring bounce and complaint webhooks until suppression breaks.
- Treating an email API as “fire and forget” with no event logging.
Sources & Further Reading
Official docs for current setup details, pricing, and API behavior — verify specifics there, since they change.
Related guides
More on smtp relay for saas and the surrounding transactional email workflow:
FAQ
What is an SMTP relay?
A third-party service your app connects to over SMTP to send email, so you don't run your own mail server. It handles delivery and IP reputation — you just hand it the message. Providers like SendGrid, Mailgun, Brevo, Amazon SES, and self-hosted useSend all offer one.
When should a SaaS use an SMTP relay instead of an API?
Use a relay when you're integrating software that only speaks SMTP, or when you want portability — swapping providers becomes a config change rather than a code rewrite. Use an API for new backend code that wants richer features and clearer errors. Many teams use both.
Do I still need SPF, DKIM, and DMARC with an SMTP relay?
Yes. A relay delivers your mail, but inbox placement still depends on authenticating your sending domain with SPF, DKIM, and DMARC, plus handling bounces and complaints. The relay doesn't remove that work — it just spares you from running a mail server.
Which SMTP port should I use?
Use a TLS-secured submission port — 587 with STARTTLS, or 465 for implicit TLS — and never send over an unencrypted connection. Port 25 is for server-to-server delivery and is often blocked for app submission, so it's not the right choice for relaying from your app.