What Is an SMTP Relay and How Does It Work?

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smtp relay and How Does It Work?
Sep 23, 2023

We all think that sending emails is too easy but it is true when we send emails one by one. The more tricky is sending bulk emails and setting up email automation. There are many complicated things like SMTP servers, PMTA, DNS, IMAP, and other things. 

There are too many tools available to set up your email marketing campaign or automation. The SMTP server is responsible for sending, authenticating, and receiving emails. Today, we are going to understand the SMTP relay. Is the working of the SMTP relay & SMTP server is same and most importantly why do you need it for your business?  

What is an SMTP relay? 

The SMTP stands for simple mail transfer protocol and SMTP relay is a protocol that works as an intermediary for transferring email messages from one SMTP server to another. It receives ongoing email messages from email clients and then forwards or “relays” them to the intended recipients’ email servers.  

For example: Imagine a company that has multiple branch offices worldwide. They can use an SMTP relay server located at their headquarters to ensure consistent email delivery. When an employee sends an email, it goes to the relay server first, which then forwards it to the recipient’s email server.

An SMTP (Simple Mail Transfer Protocol) server is a specialized computer or software that sends, receives, and routes email messages over the Internet. It’s responsible for delivering outgoing emails from email clients to the recipient’s email server.

Now we are going to understand how it works.

How does the SMTP relay service work?

SMTP relay service works as an intermediary protocol between the sender’s email server or client and the recipient’s email server. Here’s how it works:

Sender’s Request: A sender initiates an email using an email application. This email application connects to the SMTP relay server.

Email receives: This email provides all the required credentials to the SMTP relay server that is used in the authorization of email. This allows the relay server to process the email.

Relay Server Processing: Now, the SMTP relay server checks all for errors to process it. This SMTP server looks into a domain name server to identify the recipient addresses, formatting, and security measures.

Forwarding: After processing, this server sends the email to the email server of recipients using the recipient’s domain information.

Recipient Server Delivery: The recipient’s email server receives the email from the relay server and stores it in the inbox.

Why do Companies need an SMTP relay service?

SMTP server plays a crucial role in improving email deliverability, security, and email reputation. This SMTP relay service helps in sending bulk emails. 

Widespread usages

The STMP server is the most trusted protocol for sending emails. Companies widely use it to fulfill all their email needs including bulk email, personal email, and email automation. This also offers email APIs to use it. 

SMTP relay services help businesses prevent domain blocklisting while sending marketing emails. 

Higher Performance: 

SMTP relays help to distribute email traffic efficiently to prevent overloading on the company’s email server.  SMTP relays ensure consistent performance and compliance with local regulations.

SMTP relay services help businesses send effective email messages without relying on their own servers and domains.

Authentication and Security: 

SMTP servers support all email authentication protocols like SPF, CCPA, DKIM, and others. This also provides Transport Layer Security (TLS) to encrypt all your email transferring communication. 

There are many email software for user like Outlook and Thunderbird offers user-friendly interfaces for SMTP relay. Email’s vulnerability has led to security measures like SSL/TLS, SPF, DKIM, and DMARC being integrated with SMTP relay for enhanced cybersecurity in internet communication.


You can analyze the performance of the email marketing campaign using an SMTP server. It includes click rate, open rates, delivery rate, and other things. 

You can analyze how many emails are delivered to the recipient’s inbox and how many emails are bounced. This SMTP server sends a notification every time an email is bounced. This helps to maintain a clean email list to ensure higher email deliverability. 

Higher Email Deliverability: 

SMTP relay allows businesses to send messages across the internet without relying on an internal server which helps in reducing the burden on their infrastructure. SMTP relay services offer opt-out and unsubscribe features to maintain a proper clean email list. This is important to avoid domain blocklisting of a business. 

SMTP relay port

Selecting an SMTP relay port is another tricky part of email marketing. This port helps to facilitate communication between email servers over the internet. They ensure your email messages are securely delivered to their intended recipient.

The port number is important to connect your email from the SMTP server. An SMTP port is known as a communication endpoint that is responsible for managing the transfer of email data via SMTP. Different SMTP ports are used for different purposes like standard email transmission, secure encryption, and alternative options.

The best way to select an SMTP relay port is to understand your requirements and budget. There are many commonly used ports including 25, 465, 587, and 2525 that are always open for use. Port 25 is standard but may have limitations, while ports 465 and 587 offer encryption for added security. There are other ports are also available to ensure reliable email delivery. 

But the more important to understand that each port does not work in all situations. For example, Port 25 is a standard mail server communication that is frequently blocked by ISPs and cloud providers. 


SMTP relay is essential for sending email messages efficiently and reliably. This works as an intermediary for sending email messages from to intended recipient. SMTP relay service offers benefits like sending bulk emails, maintaining a clean email list, higher email deliverability, and preventing domain blocklisting. You can send messages over the internet without the need for an internal business server to handle them. 

Mailerday is a Perfect Email Marketing Service provider to grow your business. You can either hire our Email Expert or use our email services including SMTP service, email verification, a bulletproof server, & DMCA-ignored bulletproof service. All services are affordable and have 24/7 support.


Is an SMTP relay the same as an SMTP server?

No, the SMTP relay and SMTP server are not the same. SMTP (Simple Mail Transfer Protocol) is a communication protocol that is used to send, receive, and route email messages. However, an SMTP relay is a service that acts as an intermediary for email transmission between SMTP servers. SMTP relay service helps to improve email delivery reliability, scalability, and security.  

Are there any limitations or restrictions on the volume of emails that SMTP relay services can handle?

The limitations on email volume can vary depending on the SMTP relay service provider. Many providers offer different plans with varying email-sending limits. You need to understand your requirements, budget, and goals to choose a perfect plan. 

Can SMTP relay services be used for both transactional and marketing emails?

Yes, SMTP relay services are a versatile option that can be used for both transactional and marketing emails. This ensures to maintain domain reputation of a business. It is best fit with different types of email communications like order confirmations and promotional marketing emails.

Can I configure multiple SMTP relay servers for better performance and load balancing?

Yes, configuring multiple SMTP relay servers is possible and can improve performance and load balancing. This setup distributes email traffic across multiple servers that reduces the risk of overloading on a single server. This is common for high-volume email senders.

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