SMTP Relay
An SMTP relay (also called a smart host) is an intermediary SMTP server that accepts outbound email from your application and delivers it to recipients on your behalf. SMTP relays improve deliverability by sending through pre-warmed IPs with established reputation, rather than from your own server's IP.
When to Use an SMTP Relay
Use an SMTP relay when your application or platform (WordPress, Laravel, Django) already speaks SMTP and you want to improve deliverability without rewriting code to use an API. Common scenarios:
- ●Legacy applications that only support SMTP
- ●CMS platforms with built-in SMTP configuration
- ●Quick migration from self-hosted mail servers
- ●Development environments where SMTP is easier to configure than API integration
SMTP Relay Configuration
Most SMTP relays require four settings:
- ●Host: The relay server hostname (e.g., smtp.xmit.sh)
- ●Port: Usually 587 (STARTTLS) or 465 (SSL/TLS)
- ●Username: Your account identifier or "apikey"
- ●Password: Your SMTP password or API key
Configure these in your application's email settings and your outbound email will route through the relay's infrastructure.
Related Terms
SMTP vs API
Two approaches to sending email: SMTP uses the legacy mail transfer protocol, APIs use modern HTTP endpoints.
Email API
A programmatic interface for sending, receiving, and managing emails from your application code.
IP Reputation
The trustworthiness score assigned to an IP address based on its email sending history.
Need help with email deliverability?
Transmit handles authentication, warmup, and reputation isolation automatically.