How does a mail relay server work

A mail relay server, also known as a mail relay or mail hub, is a server that acts as an intermediate step between a mail client (such as a mail user agent) and a mail server (such as a mail transfer agent). Its primary function is to forward email messages between the two.

Here's a high-level overview of how a mail relay server works:

Mail Client → Mail Relay Server

1. A mail client (e.g., a user's email client software) sends an email message to the mail relay server.
2. The mail client connects to the mail relay server using the Simple Mail Transfer Protocol (SMTP).
3. The mail client authenticates with the mail relay server using a username and password or other authentication mechanisms.

Mail Relay Server → Mail Server

1. The mail relay server receives the email message from the mail client and checks its configuration to determine where to forward the message.
2. The mail relay server looks up the destination email address in its database or uses a DNS lookup to determine the mail server responsible for the recipient's domain.
3. The mail relay server establishes a connection with the destination mail server using SMTP.
4. The mail relay server forwards the email message to the destination mail server.

Mail Server → Mail Relay Server (optional)

1. If the destination mail server is not available or the message needs to be stored temporarily, the mail relay server can store the message in its queue.
2. When the destination mail server becomes available, the mail relay server can retry sending the message to the destination mail server.

Mail Relay Server → Mail Client

1. When a mail client sends an email message to the mail relay server, the mail relay server can store the message in its queue.
2. When the destination mail server becomes available, the mail relay server can forward the message to the destination mail server.
3. The destination mail server can then deliver the message to the recipient's email client.

Mail relay servers can provide various benefits, such as:

• Load balancing: By distributing email traffic across multiple mail relay servers, you can improve the performance and reliability of your email infrastructure.
• Spam filtering: Mail relay servers can be configured to filter out spam and malware before forwarding email messages to the destination mail server.
• Authentication: Mail relay servers can authenticate email messages using mechanisms like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance).
• Backup and disaster recovery: Mail relay servers can store email messages in their queue, allowing you to recover email messages in case of a disaster or outage.

In summary, a mail relay server acts as an intermediate step between a mail client and a mail server, forwarding email messages between the two while providing additional features and benefits.