When you send one email, a sender interacts with a recipient simultaneously. But when you deal with transactional emails or bulk mails in your app, they can't all be sent at once. Emails are put on an SMTP queue that provides temporary storage before processing.

Meaning of an email queue

You first have to ask Yourself what a queued email is. Email queuing is a form of Message Queuing – an asynchronous service-to-service communication. An email queue decouples the sender from the recipient. As such, the queued emails wait for processing until the recipient is available to receive them. The process is meant to decouple a producing process from a consuming one.

An email queue is stored before they reach the SMTP server. The communication between the sender and the recipient is asynchronous. Once the emails have been enqueued, they're delivered step by step to the end user via SMTP.

How email queues work

In this part, we will understand how to send a queued email. Most mail servers use a mail transfer agent (MTA) called sendmail to do the actual sending. The MTA will periodically send all the messages in the queue until completion. If the recipient's SMTP server does not respond, Sendmail will recurrently send emails.

Why Email Queues Get Clogged And How To Fix Them

When doing a mass mailing, the SMTP server puts your outgoing messages automatically into an email queue. They are sent one by one from this buffer. Queued emails won't be bounced. They'll be sent anyway, but the delivery time can be significantly reduced. Once you have learnt what a queued email is and how to send a queued email, we will discuss how to fix queued emails that have been clogged down below.

Exceeded volume of emails

Some email providers enforce email rate limits on IP addresses. If you exceed this rate and queue too many emails, the delivery speed will decrease. The only solution is to contact the incoming server as frequently as possible. Also, it's advised to opt for a dedicated IP address instead of a shared option.

Another common reason is that spam filters have busted your email. The filter will let the emails pass to analyse how the rest of the recipients react. If there is slow progress, it's okay. Your email campaign is being observed and assessed. Please read our blog post on how to avoid emails going to spam.

Ways of managing email queues

Managing an email queue is a simple task if you use a server control panel like cPanel. The most widely used mail transfer agents are Postfix and Exim. CPanel provides WebHostManager (WHM) to handle a bunch of stuff for you.

Email queue in your app

If your app tracks megabytes per month, you should opt for a refined email sending architecture. It's based on an asynchronous system to send emails, queue messages before they reach the mail server. That's how it may look at a higher level:

There are processes involved that make this process happen, and they include ;

Email queue is implemented via the email queueing system

What happens when you use an email queuing system like ActiveMQ or RabbitMQ. It's not an SMTP queue. Emails are put on a line but not on the mail server. Without a row, your app will try to send out thousands of emails at once.


We have learnt about what queued emails are and how to send queued emails amongst other things. So look into them keenly so that in case you use this feature you can notice an issue and you can solve it as fast as possible.