An SMTP server (Simple Mail Transfer Protocol) is the dedicated server that handles outgoing email. When your application, CRM or website sends a message, it talks to an SMTP server, which authenticates the request, queues the message and delivers it to the recipient’s mailbox provider. An SMTP relay is a managed SMTP service designed to send transactional and marketing emails reliably at scale — without the headaches of running your own mail server.
How an SMTP relay works in 4 steps
1. Your app connects to the relay server (e.g. smtp.mailpro.com:587) over TLS. 2. It authenticates with username and password (or API token). 3. It hands the message to the relay. 4. The relay signs it (DKIM), routes it through a high-reputation IP and tracks delivery, opens and clicks.
SMTP relay vs other sending options
| Option | Best for | Limits |
|---|---|---|
| Personal SMTP (Gmail, Outlook) | A few personal emails per day | ~500/day, no transactional, low deliverability |
| Self-hosted Postfix | Full technical control | You manage IP reputation, blacklists, queues yourself |
| SMTP relay (Mailpro) | Transactional + marketing at any volume | Pay per email or monthly plan |
| Email API (REST) | Tightly integrated apps | Requires SDK or HTTP code |
The right SMTP port
| Port | Protocol | Recommended use |
|---|---|---|
| 25 | Plain SMTP | Server-to-server only — usually blocked by ISPs |
| 465 | SMTPS (implicit TLS) | Legacy compatibility |
| 587 | Submission with STARTTLS | Recommended for client-to-relay |
| 2525 | Submission alternative | Fallback if 587 is blocked |
Mailpro accepts ports 587 (recommended), 465 and 2525.
Why use a managed SMTP relay
Managed relays handle the four hardest parts of sending email at scale: maintaining IP reputation, configuring SPF/DKIM/DMARC, dealing with bounce-back loops and following ISP rate limits. They typically achieve 98%+ inbox placement out of the box, which is unrealistic for a self-hosted server without a dedicated team. Read more in our blog post on what is SMTP relay and SMTP vs email API.
Setting up Mailpro’s SMTP relay
1. Open Account → SMTP credentials. 2. Generate a username and password. 3. Use them in your application’s SMTP settings. 4. Authenticate your domain with SMTP authentication for higher deliverability. Detailed steps: how to send emails with SMTP server.
Send transactional email at scale
Mailpro’s SMTP relay handles authentication, deliverability and tracking out of the box. Read the full SMTP relay FAQ or visit the SMTP page.
Related: how to integrate Mailpro with WordPress.