An SMTP relay service is a managed outgoing mail server that sends email on behalf of your application, website, CRM or email client. Your software hands each message to the relay over SMTP; the relay authenticates you, delivers the message to the recipient's mail server from IPs with a trusted reputation, retries when needed and reports what happened.
How does an SMTP relay work?
- Your application connects to the relay host (for Mailpro,
smtp.mailpro.com) on port 587 with STARTTLS, or 465 with SSL/TLS. - It logs in with an SMTP username and password, so only authorized senders can use the relay.
- The relay accepts the message, looks up the recipient domain's mail server and delivers it from a reputation-managed IP, authenticated for your domain once SPF and DKIM are in place.
- If the recipient server is temporarily unavailable, the relay retries; bounces are recorded so your lists stay clean.
- Delivery, opens and clicks are logged in your statistics.
SMTP relay vs SMTP server
Every relay is an SMTP server; "relay" describes the job of accepting mail for domains it does not host and passing it on. An open relay accepts mail from anyone and is quickly abused and blacklisted, which is why a legitimate SMTP relay service only relays for authenticated users.
Why use an SMTP relay service instead of your own server?
- Deliverability: web hosts and office servers often hit port 25 blocks, rate limits and poor IP reputation. Personal Gmail or Outlook accounts also cap daily sends (around 500 a day for a free Gmail account).
- Volume: a relay is built to send large batches and transactional email without being throttled.
- Authentication: SPF, DKIM and DMARC are set up once for your domain.
- Visibility: per-message delivery, bounce, open and click statistics.
- Less maintenance: no queues, patches or blacklist monitoring to run yourself.
Sending from WordPress, a CRM or your own app? Mailpro's SMTP sending servers connect in minutes with the credentials in your account, no code required.
Which port should you use?
| Port | Encryption | Use it for |
|---|---|---|
| 587 | STARTTLS | Recommended for apps and email clients |
| 465 | Implicit SSL/TLS | When STARTTLS is not available |
| 25 | None or opportunistic | Server-to-server only, often blocked by ISPs |
| 2525 | Same as 587 | Non-standard fallback offered by some providers |
For Mailpro, use 587 or 465. Details in which SMTP port to use.
What to look for in an SMTP relay service
Check authentication support (SPF, DKIM, DMARC), how IP reputation is managed (shared pools or a dedicated IP), per-message tracking and bounce handling, the pricing model, where your data is hosted, and how quickly technical support answers. Compare options in what is an SMTP relay, SMTP relay vs email API and the best Amazon SES alternatives.
How to set up Mailpro's SMTP relay
Find the host, port, username and password under Settings → SMTP in your Mailpro account, enter them in your software's outgoing-mail settings, then add SPF and DKIM for your sending domain. Step by step: how to send emails with the SMTP server and SMTP authentication. Using WordPress? See how to integrate Mailpro with WordPress.
Mailpro and SMTP
Beyond the relay — every transactional email tracked
Mailpro's Swiss-hosted SMTP relay sends from reputation-managed IPs and logs delivery, opens and clicks for every message in its own statistics dashboard.