Understanding SMTP email queues

Agathe Brusset Agathe Brusset
· 6 min read · Tips and resources · August 26th, 2021
An SMTP email queue works in the background to deliver your emails by sending them one at a time. You can take measures to reduce your email backlog, but ultimately your choice of an SMTP email provider determines your email deliverability.

We take for granted that emails are sent instantly after we click send. But what if email servers had to complete sending the current message before dispatching the next one? Emails would crawl to a standstill as people patiently wait for their turn!

Thankfully, this situation is avoided by using SMTP email queues. An email queue enables asynchronous communication by creating a buffer of outgoing emails. Plus, sendings can be scaled as the server doesn’t need to wait for a response before sending the next email.

Read on to learn more about how an SMTP server queues your email for maximum deliverability.

What is an SMTP Server?

SMTP (Simple Mail Transfer Protocol) is the go-between for email providers like Gmail and Yahoo. When you send an email message, it becomes a line of code. A Mail Transfer Agent (MTA) processes this code and figures out the recipient of your message. Once the other server receives the email, it is downloaded and delivered to your recipient.

SMTP servers can also perform other tasks. If receiving servers are blocking your messages, for example, the server returns the emails to you along with non-delivery reports. In addition, webhooks can notify you when emails are not properly received by recipients. You can also add authentication records to reduce the risk of spam.

All of these SMTP processes and more happen quickly in the background. However, if you try to send a few hundred emails at once, you might end up with a backlog of queued emails. Let’s dive deeper to understand why this happens!

How email queueing works

The SMTP server from MailerSend can arrange emails and send them one at a time to ensure the deliverability of your messages. This is known as an SMTP queue. The server temporarily stores the emails while it processes each one individually and then sends them to the recipient.

The benefit of an email queue is that your emails will not bounce or get deleted while they are waiting their turn to be sent. However, you might have difficulty sending a large batch of emails if the message queues keep getting clogged with emails.

Managing email queues on your own can be hard, this is why ESPs like MailerSend are here to help you. You can focus on your business operations while MailerSend takes care of your email delivery.

Causes of backlogged queues

Some types of emails are more prone to delays than others, contributing to a backlogged queue. Typically, bulk emails like these examples could cause sending delays:

  • Newsletters. If you’re trying to send a newsletter to thousands of subscribers at once, you'll probably experience a gridlock. This could cause problems if you need to send the newsletter by a certain time. A marketing email service provider like MailerLite will queue your newsletters and ensure that subscribers receive them.

  • Receipts. When many people make purchases at the same time, say on Black Friday, the system sends a confirmation email to each person. Some people might not get their receipt right away if the system is backlogged with shoppers!

  • Social media updates. A single person could get dozens or hundreds of social media notification emails a day, which will quickly overwhelm mail servers. The queue worsens when thousands of people sign up for email notifications.

  • Password changes. Password reset emails are regularly sent to help users gain access to their accounts. But if many people request a new password at once, the emails might go into the queue and frustrate waiting users!

That’s why you should separate your marketing emails and transactional emails, avoid using the same sending server or, in some cases, the same domain name.

A best practice would be to use MailerSend's SMTP server for your transactional emails and MailerLite for marketing-related communications.

Resolving SMTP email queues

If you have a backlog of emails waiting to be sent, you could wait for the SMTP server to finish sending. However, recipients may be waiting to receive time-sensitive transactional emails like webinar login details. Here’s a quick troubleshooting guide for email server connectivity issues.

1. Use email queuing functionality

If you send thousands of emails every day, an email queuing service offered by email service providers like MailerSend gives you more control over the process. A regular SMTP queue might not be able to handle a high volume, non-stop flow of outbound emails.

Email queuing functionality puts emails in a queue before they reach the SMTP server, sending emails to the server in batches. When you reduce the number of messages sent to a server, the risk of email backlogs is decreased and the chances of delivering emails on time are increased.

2. Contact the server

Major email services often have rate limits attached to your IP address. If you try to send too many emails at once, the provider will slow the delivery rate. This prevents spam from being sent but it can be frustrating when you have a legitimate reason! 

If you were to use your own IP address, you would have to apply rate limits to match the requirements of common ISPs. But finding those requirements can be hard, that’s why it’s better to leave deliverability to an experienced provider so you don’t need to worry about it.

3. Avoid spam filters

If you suddenly send emails with a new IP address, mail servers might think that you’re a spammer and you may even be blocked! You first need to build a positive email reputation with your domain and IP address to prove that you're a genuine sender. Otherwise, your recipients might not even see their inbound emails—they'll go straight to the spam folder.

You can avoid the hassle of gradually warming up your sending domain by using an SMTP relay service. Plus, MailerSend takes care of your IP reputation for you by automatically adding hard-bounced and unsubscribed recipients to suppression lists. This way you won’t accidentally email them again!

MailerCheck helps you scan your emails for deliverability red flags before you send. Email Insights is an inbox test that puts your email content through a spam filter, checking if email headers, like your subject line, contain spam-like words.

4. Check firewall settings

Aggressive firewall settings on your SMTP server can sometimes cause a backlog of emails. Third-party security products can block legitimate traffic, causing receiving servers to return temporary SMTP response codes. When this happens, emails are placed back into the queue for another retry.

Let your provider handle email queues

Email queues are essential to SMTP email because they enable asynchronous communication and sending of email at scale. You can take measures to resolve a backlog of emails but, better still, choose an SMTP relay service that is optimized for deliverability and has a stellar reputation!

How do you handle a sending backlog of emails? Share in the comments below.

Agathe Brusset
I'm Agathe, Product Manager at MailerSend. When I'm not busy rolling out new product features, I'm planning my future mud-brick farmhouse complete with farm animals and a vineyard!