Email Delivery Failure: Why Didn't Customers Receive Your Email?

Why didn't customers receive your email?
Yurii Bitko Yurii Bitko 30 august 2023, 14:29 244
For beginners

Poor email delivery rates can be maddening. You spend time and effort creating a marketing email campaign for your customers, only to see that lots of messages don’t get to their intended recipients.

Don’t fret if you’re struggling with undelivered emails. The problem has its cause and possible solutions, and it’s essential to understand the cause before finding a remedy. This article will discuss the most common causes of unsuccessful email deliveries.

Causes of email delivery failures

Email delivery failures can occur for diverse reasons. It could be that your domain has high spam complaint rates, causing email services to block your messages. Your email servers may be misconfigured and unable to send messages correctly. Some addresses you sent messages to may be invalid. The possible causes are endless. The most common ones to note are described below.

Client Side Issues

The first set of issues you may encounter relates to a variety of problems with your own computer or software. Let’s look through a few scenarios.

You’re not online

The problem could be as trivial as a failed internet connection; it may have dropped unknowingly, making you unable to connect to your mail server and send messages. A simple check will confirm if that’s the case.

Sometimes, you could be connected but still have some issues with your internet service provider (ISP) that cause your connection to malfunction. In that case, try to restart your connection to see if it works.

Setting up an email client with the wrong SMTP parameters

You may have entered incorrect SMTP parameters when setting up your email client. In such cases, your mail client won’t be able to communicate with your server, causing an immediate failure. Check the parameters to see if you made a typo and correct it.

Wrong SMTP port

An SMTP port number is an important parameter of your email client setup. Using the wrong SMTP port means it won’t work. Check with your email provider or network admin to be sure that you specify the correct value and verify that it is also supported by your client software.

Firewall settings

Antivirus or firewall settings could prevent messages from leaving your computer and getting to the SMTP server. Similarly, the recipient’s firewall might block your incoming messages based on its defined rules. See your firewall’s logs to find out if this is the case.

If this issue occurs, you can set up an exception for your firewall to allow SMTP connection.

Malfunctioning SMTP server

The issue could be related to your email infrastructure, either in-house or external. Network misconfiguration or a hardware failure can prevent mail servers from sending messages to their intended recipients. Your email client parameters may be set up correctly, but the SMTP server itself may be malfunctioning. If you’re using an external provider, contact customer support to rectify the issue. If you operate your own server, you’ll have to troubleshoot it by yourself.

We can’t overemphasize the point of choosing a suitable SMTP provider like UniOne. They take care of the infrastructure and give you peace of mind to send messages. Making a bad choice means you might experience significant downtime, preventing your emails from reaching their targets.

Receiving Server Issues

An even larger list of problems relates to your addressee’s SMTP server. It also may be improperly set up or just unwilling to receive your email for certain reasons. Let’s take a closer look at this category.

Temporary server problems

Temporary mail server issues can cause email delivery failures. For instance, the recipient’s server could be under maintenance and thus unable to receive messages. The message would be delayed and may end up undelivered. You can retry it once the server is back on.

In this scenario, your sending server will probably receive a “soft bounce” notification, telling it that an additional attempt to send the message may be taken in a short time. Most servers will retry sending the message automatically.

Invalid email address

Invalid addresses represent the most common cause of email delivery failures. It’s impossible to send messages to addresses that just do not exist.

People often make typos when entering their email, so check if that’s the case immediately after a delivery failure notification. Scan every character in the address to find the error and correct it. Look for common typos, such as instead of

The invalid address might have existed before but was later changed or deleted by the owner. For instance, if someone leaves their workplace, their work address may be removed. Sending further messages to this address won’t work.

The addressee’s domain might also be the issue. Its mail server could be down, rendering mail exchange impossible. It could expire, and this will lead to permanent failure.

The good news is that email service providers usually alert you when an invalid address is the reason for delivery failure, automatically blocking permanently unreachable ones from future mailings.

Full mailbox storage

Your recipient may have exceeded their mailbox storage limits, and your message has no place to stay. It’ll bounce back along with a “mailbox storage full” or similar notification. Most mailbox providers offer enough space to everyday users, so this error is uncommon.

You can hardly do anything to fix this issue except to somehow tell the recipient to delete unneeded files from their mailbox. This might be possible if you also have their phone number or messenger id.

Spam filters

Spam is the biggest problem on the internet; about half of all emails constitute spam. Hence, mailbox services use spam traps and filters to protect users. Sometimes, a legitimate message can get caught up in a spam filter, preventing it from reaching the intended recipient.

You can overcome this problem by creating relevant content and avoiding behaviors associated with spammers. For example, spammers are known for excessively using exclamation marks and having lots of typos. Ensure your message is well-written and relevant to your target audience, and you’ll avoid falling into spam filters.

High number of spam complaints

Email users can mark any message as spam. Mailbox providers keep track of the number of such messages from a specific domain or IP address. If the numbers are too high, they restrict or block the sender to prevent further complaints.

Many of your messages won’t be delivered to the intended recipients if you have a high spam complaint rate. Instead, they’ll be bounced or placed in a spam folder where no one would read them.

An ideal complaint rate is less than 0.1%. Anything above this figure can eventually cause delivery failures. You can avoid high spam complaints by sending legitimate messages to addresses whose owners have consented to receive them. Another important measure is removing contacts that do not open your messages for a long time.

Permission issues

This failure scenario is a sub-type of the previous one. A good email list is built on solid permission standards. You must obtain explicit permission from the owners of the addresses on your mailing list before sending messages. Sending messages without permission won’t work; they’ll be largely marked as spam, and mailbox services will eventually blacklist your domain.


The domain or IP address you’re sending emails from could be blacklisted by mailbox providers, causing your message to fail. Spam complaints are the most common reason for such blocks, so keep your spam complaint rate at a minimum, and you’re good to go.

Sometimes, you may happen to share an IP address with a malicious actor sending spam messages and get blocked along with everyone using that IP address. You can solve this issue by getting a dedicated IP address from UniOne or any other provider. This way, you retain full control over your domain/IP reputation.

Attachment limitations

Avoid including attachments in your emails as much as possible, as they often cause problems. If your attachments exceed the size allowed by the receiving server, it’ll cause the email to fail. Most mailbox providers allow a maximum of 25MB or less for attachments. They also prohibit certain attachment file formats, such as executable files (.exe).

Besides, attachments are potential vectors for spreading malware, so people are wary of receiving them from addresses they are not familiar with.

If you really need to send a large attachment, don’t fret. Try using a compression tool to reduce the size before sending it. For a better solution, you can upload the file to a file-sharing service like Google Drive or Dropbox. Then you just send a link to the file instead of the entire attachment.

Sender authentication problems

You can use email authentication techniques to sign your emails electronically. The electronic signature certifies that a message comes from your authorized domain/IP address and not that of an impersonator.

Any problem with your authentication setup can cause delivery failure. Misconfigured authentication records will make mailbox providers think you’re an impersonator and bounce your messages.

Recipient’s settings

Email users can set their own specific rules for their incoming mail and control the messages they receive. For example, someone might set their email client to block any message with an attachment automatically.

Your message could get blocked due to the recipient’s custom settings, which you cannot control.


Sooner or later you’ll likely experience email delivery failure and wonder what’s causing it. Troubleshooting the issue might be difficult because you don’t know where to start. We have explained the most common causes of email delivery failures and tips to resolve them.

Follow our tips, and you’ll likely solve most email delivery issues you encounter. Above all, be sure to choose a suitable email service provider to guarantee the delivery of your emails. UniOne is one such provider offering a reliable email delivery service for an affordable price.

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