My email failed to send: Understanding the error message

Let’s say you’ve integrated your app with email functionality, designed some great email content, developed a marketing strategy, and you think you’re good to go. But then you hit send from your email client, and you see one of the following error messages from your mail delivery subsystem:

• Message not delivered.
• Email failed to send.
• Failed to send email.

You’ve probably thought long and hard about emails failing to get delivered, but what’s this about emails failing to send? In this article, we discuss some of the common causes for undelivered emails. Then, we share some ways to fix the issue and leave you with five tips for smoother future email sends.

Simply put, this message is as it says: your email is currently not deliverable. Email delivery failure can feel complicated, but the first place to look is at your SMTP server, the mail server responsible for sending, receiving, and transferring your email via the Simple Mail Transfer Protocol (SMTP) protocol. SMTP then uses TLS/SSL to secure the network connection between your email client and your mail server.

Email sending failures often occur if you self-manage your own SMTP server, which comes with large technical overhead and security requirements.

Don’t give up too easily – just because your email failed to send this time doesn’t mean you’ve stumbled into an unfixable situation. The reasons your email failed to send vary from the mundane (bad internet connection) to the technically complex (server issues – like your SMTP server connection).

Here are some common reasons your email messages won’t send:

1. You’re not connected to the internet.
2. You’ve set up your email client with the wrong SMTP parameters.
3. Your SMTP server connection is malfunctioning.
4. You’re using the wrong SMTP port.
5. Your firewall settings are getting in the way.

Let’s unpack each of these below and walk through some troubleshooting steps.

Have you tried turning it off and on? Okay, we don’t mean to make light of the situation, but sometimes this happens. Check your internet connection first before tearing apart your SMTP server setup. Sometimes, the issue really is just that simple.

Are your emails not sending in Microsoft Outlook or Gmail via your app integration? Chances are you’ve set up your webmail client with the wrong SMTP authentication or made a few typos.

Okay, this is where it gets tricky. Your SMTP server connection may be malfunctioning. If your SMTP parameters are correctly set up, your email server may be down or not functioning.

The old school convention states that your outgoing mail server should use port 25. However, some inbox service providers (ISPs – not to be confused with internet service providers) block port 25 because of spam issues.

That’s right, check your antivirus or firewall settings. Sometimes, outgoing email can be blocked by your own privacy protection protocols.

The best way to fix a problem is to prevent it. With that being said, here are five ways to avoid that dreaded “email failed to send” message:

1. Check your internet connection and email password. Typos and human errors are easy. Save yourself a few headaches by making sure your Wi-Fi connection works and that your authentication protocols are all properly set up before you hit send.
2. Check your SMTP server details. Ensure you’ve correctly entered the right parameters and that your mail server is functioning as intended.
3. Use the correct SMTP port. Port 587 is considered the default SMTP port for mail transfer, and it’s what you should use unless you’re blocked by your network or internet provider.
4. Use an ESP that provides an SMTP relay service. ESPs, like Mailgun, provide an SMTP relay service that helps you circumvent all the headaches associated with running your own SMTP server. Leave delivery failures in the dust while the pros manage your email delivery.

There are other ways to send mail, especially if you send in bulk. If you’re a programmatic sender, a failure to send error can be a much bigger deal, impacting a large volume of transactional emails. Here are some common API sending issues and troubleshooting tips to help you identify why your emails won’t send.

1. Incorrect API endpoint or credentials: Double-check that you are using the correct API endpoint for sending your emails and ensure that your API key is correct and has the necessary permissions. This happens more often than you might think.
2. Rate limiting: Does your email service provider rate limit? Some APIs have rate limits set to prevent abuse. Rate limiting can also be part of an account tier, pricing plan, or permissions level. Make sure you’re not hitting rate limits by sending too many requests in a short amount of time.
3. API configuration: Check that you’ve configured your API correctly. This includes setting up sender email addresses, recipient addresses, subject, body, and attachments.
4. Email content: Some email APIs can have content policies around their elements, (subject, body, attachment) to ensure the content isn’t built in a way that would trigger spam filters. Make sure you are following best practices to avoid this.

Sender reputation plays a big role in whether your emails make it to the inbox or get flagged as spam. With tools like Mailgun’s reputation monitoring, you can spot and fix issues before they become a problem. Verifying email addresses is another simple way to keep your list clean and reduce bounces, which goes a long way in keeping your emails on track.

“Merely meeting the authentication standard is not the thing that’s going to guarantee inbox placement – it’s just a bare minimum.”

Alison Gootee Deliverability Advocacy Specialist, Sinch Mailgun

In other words: authentication is foundational, but it’s not enough on its own. Your domain’s reputation, sending practices, and engagement metrics all contribute to your ability to land in the inbox.

Here’s a last list of quick resources if you are having other issues with sending emails:

• Authentication: You won’t get far at all without setting up your email authentications. These protocols help validate your identity as a sender, and signal to receiving mail servers and providers that your messages are trustworthy and should be let into the inbox. Learn more in our post on email authentication.
• Email bounces: When sent emails bounce back, it can be because of something temporary like a connection issue or something more permanent. Get all the details in our post on understanding email bounces.
• Validation: Keeping a clean email list is extremely important for deliverability. Why go through through all the trouble of setting up a good sending infrastructure if you’re just going to send to an invalid email address? But sending to outdated or mistyped contacts doesn’t just hurt your engagement metrics – it also sends a red flag to inbox providers that you might be a spammer. If you did obtain a recipient’s email address unethically, that’s even worse – and can land you on a blacklist. Wondering if you’re practicing good list hygiene? Check out our list hygiene tips here.

Server settings and SMTP issues are a key culprit for failed email messages, but there are lots of reasons your outgoing emails may not make their way to the inbox. Resolving “failure to send” doesn’t automatically mean everything is perfect in terms of your deliverability.

Interested in more resources? We have a lot. Be sure to subscribe to our newsletter so you don’t miss out on useful email info.

Author: The Sinch Mailgun team The Sinch Mailgun team shares news, best practices, and strategies to take your products and apps to the next level using email. Subscribe to our newsletter to get all the articles in your inbox!