When you compose an email and click on send, the email travels from your computer to your email server using the SMTP protocol. But before reaching their intended destination, these emails go through additional screening.

The email protocol

So if you are wondering what happens each time you push out bulk emails, you need to learn about the SMTP feature known as mail queue processing on a remote server. What is an SMTP email queue? Read on to find out.

What is an Email Queue?

The term "email queue" simply refers to a list of emails that are awaiting processing and delivery. When an email is in the queue, it signifies that internal issues are preventing it from being sent. Usually, these backlogged emails arrive late without causing any problems.

Email queue is an asynchronous service-to-service communication that isolates the sender from the recipient. Once the recipient's server is ready to receive it, the operation ends.

Since SMTP uses these protocols to send your emails, the email queue is a component of SMTP servers.

How Email Queues Works

Asynchronous communication is made feasible by email queues since the server doesn't have to wait for a response before delivering the next email. The SMTP server on your host adds these emails to a queue, and an MTA like Sendmail handles the actual transmission. The MTA will eventually send each message in the queue, but if the recipient's SMTP server is unavailable for an extended period, an email will be returned (for example, five days).

A mail server, SMTP server, and MTA must all communicate simultaneously for this operation to be successful. These three elements differ from one another in the following ways:

  • A mail server refers to the computer system you can send and receive electronic messages using email protocols.
  • An SMTP server is concerned with outgoing mail, which makes it where email queuing is mostly implemented.
  • Email is moved via a delivery chain by MTA software until it reaches a mail delivery agent (MDA).

Notably, Email queues can clog, which might prevent these emails from reaching their intended recipients. Let's find out what causes email queue clogging then.

Why Email Queues Get Clogged and How to Fix it

Here are the primary causes of backed-up email queues and a potential fix for each.

  • Top email providers like Gmail or Yahoo enforce email rate limits on IP addresses that should not be exceeded. The delivery speed will go down if you exceed that limit or upload an email attachment that exceeds the allowed size.

To solve this, contact the incoming server as frequently as possible to push the queue. Also, to enhance the speed of your email traffic, choose a dedicated IP address rather than a shared one.

  • Secondly, spam filters can reject your email. Your email campaign is being monitored and evaluated even though the filters will let the emails through gradually, it analyzes how the recipients react to the emails. Your IP address being blocked is just one of many potential causes if it's stuck.

If this happens, remove yourself from this list and improve your email marketing strategy.

Managing Email Queues

Managing an email queue is simple if you are using a server control panel like cPanel. This is because you will have the WebHostManager (WHM) manage the complex aspects of the process.

Here are the commands for the most popular mail transfer agents if you are not using Cpanel, though: both Exim and Postfix

Postfix Commands

  • List the queued emails:
  • Reattempt delivery of all queued emails
  • Remove all queued emails
  • Remove a particular queued email

Exim commands

  • List the queued emails:
  • Reattempt delivery of all queued emails
  • Remove all queued emails
  • Remove a particular queued email

Email Queue in Your App: Reasons to Use a Comprehensive Email Sending Architecture

SMTP servers typically feature a built-in queue management mechanism that handles email campaigns for larger recipients. But it could be ideal to have an email queue directly in your app. This will be beneficial in the following instances:

  • While performing async operations, such as sending notifications to 1000 contacts.
  • If you must send more than 10,000 transactional emails or bulk emails each day.
  • If your app monitors a monthly quota of megabytes, and you want to notify users when that quota is reached to the extent of 70%, 80%, or 90%.

Generally, Instead of sending three emails for each event, an email queue saves you stress by allowing you to send a single email per individual. Without a queue or background tasks, the process would also be extremely slow. For these tasks, the SMTP queue might not be sufficient. Because of this, you ought to select a sophisticated email sending architecture built on an asynchronous system that queues messages before they are sent to the mail server.

How to Test Your Email Queuing System

It is essential that you test this sophisticated email-sending architecture once you have built it up. Furthermore, a fake SMTP server like MailSlurp can be used to conduct these tests.

You can programmatically generate actual email addresses using the official MailSlurp Cypress plugin. You can use Cypress to send and receive emails to test user authentication, password reset, MFA, newsletters, and other features. The port, the authentication mechanism, and other credentials are all provided. You can also choose from ready-to-use integrations for the majority of well-known tech stacks.

Bottom Line

Addressing the challenges in email queueing was successfully demonstrated in this post. The operations of sending and receiving emails are decoupled by email and SMTP queuing. Although queued email frequently involves some issue that slows the message's transmission, it is effective in circumstances involving bulk or mass mailing.

Easy alternative - use a cloud email queue

Managing an email SMTP queue yourself can be very complicated. Many developer teams now use cloud email API providers such as MailSlurp that have scalable email queuing systems that offer robust controls. Try it for free today.