How Does SMTP Relay Work?
Every email relay journey follows a consistent path, regardless of the underlying technical environment. Understanding this workflow helps clarify how messages move across networks.
- The sending system connects to the relay server and submits the message.
- The relay authenticates the sender by checking credentials, IP addresses, or both.
- The relay applies any specific routing rules or policy decisions.
- The message forwards to the recipient mail server.
- The recipient server accepts and routes the message to its final destination, such as an inbox, mailbox server, ticketing system, or downstream application.
Two critical steps in this sequence deserve close attention because most email delivery issues originate there: sender authentication and destination routing.
Ports and Authentication
A port functions as a numbered communication channel that email applications use to connect with a relay server. Each port operates under specific rules regarding user permissions and security standards.
Common ports include the following configurations:
| Port | Protocol | Use Case |
| 25 | SMTP, server-to-server | Traditional server-to-server Simple Mail Transfer Protocol. Frequently blocked or restricted by consumer ISPs, hosting providers, and cloud environments for unauthenticated outbound traffic. |
| 587 | SMTP with STARTTLS | The current standard for authenticated message submission. Requires valid SMTP AUTH credentials. |
| 465 | SMTPS, SSL/TLS | Implicit TLS submission, utilized when an immediate TLS-first secure connection is mandatory. |
Routing mail through port 587 using strict authentication remains the industry standard, though legacy systems may require alternatives. This process identifies the sender to the relay before message transmission begins.
This mechanism is known as an authenticated SMTP relay. The server rejects incoming messages unless it confirms proper sender authorization. Omitting authentication creates an open relay vulnerability.
An open relay accepts and forwards emails from any source without checking credentials. Automated spam tools discover these configurations instantly, leading to rapid IP blocklisting and severe sender reputation damage.
DNS and Routing
Once the relay verifies the sender identity, it determines the correct destination for the message. Typically, it queries the Domain Name System, specifically reviewing Mail Exchange (MX) records, which identify the exact mail server responsible for a specific domain.
Many advanced relays evaluate internal routing rules first, falling back on standard DNS queries only when no custom rules match. After obtaining the correct server address, the relay connects and delivers the message.
Temporary delivery failures do not trigger an immediate cancellation. The relay automatically retries transmission according to a preset schedule, issuing a bounce notification to the original sender only after repeated failures.
