Why Won’t My Emails Send ? Guide to Technical and Deliverability Solutions

Wondering why won’t my emails send ? It can be incredibly frustrating not being sure why your emails aren't sending and ultimately disrupts your workflows, costing you revenue. Whether you’re sending a one-off email or a message with several recipients, there are many reasons your email won’t send–or appears to get blocked.

In this article, we’ll cover the most common reasons emails won’t send, as well as how to handle emails that did send, but that weren’t necessarily received by your user.

Table of contents:

• Why won’t my email send: Technical difficulties
  • SMTP Server issues
  • Bounced or rejected emails
  • Message blocked
• Why won’t my email send: Deliverability issues
  • Sender reputation
  • Engagement
  • Content
• How to ensure your emails send
• Future Email Sending Success

Why won’t my email send: Technical difficulties

If your emails won’t send, i.e. they’ve bounced, there are usually two core reasons:

1. Issues with the sender or recipient server 
2. Issues with the email address you’re targeting

In the case that you’re seeing error messages like “Message not delivered” or “Failed to send email”, this can indicate issues ranging from the Simple Mail Transfer Protocol (SMTP) server you’re using, which is essential for sending, receiving, and transferring emails, to the email server registering your email as spam, and therefore not sending it.

SMTP Server issues

If you’re receiving a message like this one, there’s a good chance that the sending issues are related to your SMTP server settings.

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In order to address this issue, you need to take a look at the following:

1. Your SMTP server details may be incorrect

Issue: Misconfigured SMTP server settings can block email delivery.

Fix: Verify that your SMTP parameters—server address, port, and authentication details—are accurate. Most email providers, like Gmail, have documentation detailing these settings, and it’s important to follow these guidelines, especially if you’re using third parties to send emails, as some apps may be considered ‘less secure’ in the eyes of Google.

2. You’re using a blocked SMTP Port

Issue: You may be using out-of-date ports such as 25 (closely linked to spam) or 465 (depreciated), which is likely to be blocked by your email service provider (ESP) or firewall.

Fix: Use an alternative Port, like 587, which is now considered the default for secure email sending.

Alternatively, there may be sending issues if you’re using an outdated version of your Email Clients or Sending Software, so always keep your email client and operating system updated. Modern software ensures better compatibility and security when sending. If you believe your software is outdated, you should contact your IT team for support.

If your SMTP settings are correctly configured and your email still won’t send, there is a chance your messages are being rejected by a server.

Bounced or rejected emails

If you’re receiving a message like this one, there are several reasons your message is not being sent.

1. The email address you’re sending to doesn’t exist or the recipient's email server couldn’t be reached

Before sending an email it’s important to ensure not only does the address exist, but also that the owner of the address's email activity will not hinder your email-sending activities. Too many bounces can lead to your emails being permanently marked as spam and blocked in the future. This is where list hygiene comes in. 

Before sending emails, it’s important to always carry out a risk assessment of the email lists you’re going to target, using the likes of a Safety Net tool. 

This will allow you to validate whether the email address is correct (or not) and test whether the address is likely to cause you issues. Even valid email addresses that won’t bounce can still cause you harm and a risk analysis will tell you which of your contacts will bounce, are disengaged, or are even likely to report you as spam.

2. Your email has been flagged as spam

If your email won’t send because it has been flagged as spam you need to take a look at the email content itself. Sending emails that contain images, salesly language and too many links is a sure way to set off spam filters.

When sending emails for the first time it’s essential to follow cold email best practices, including keeping subject lines short, and limiting the number of links to 2 max. (this also includes the Footer).

If you’re not sure which elements of the content are flagging your emails as spam, then it’s good practice to run your messages through a content spam checker.

3. You’ve reached your daily email sending limit 

If you’re sending out many emails a day, especially in bulk, you must be conscious of the daily sending limits. This varies for each email provider, for example, 500 daily sending limit for Gmail accounts and 5,000 for Outlook.

It’s crucial to note that if you send to spam traps or receive spam reports, your domain will get flagged as ‘suspicious’ by email providers and your daily limit will even be lowered further. Continually ignoring the sending limits will ultimately lead to your account being suspended, which is a sure way to ensure your emails don’t send. This is why we've written guidance on safe sending limits for sales and marketing emails vs general communications.

Message blocked

If the case is that your email is being blocked, or you receive a specific bounce code, then you need to thoroughly investigate the cause and preventions. 

ESPs like Gmail provide a list of their SMTP and other error codes, so you can get to the root cause of why your email won’t send and fix it. 

One of these issues might be related to your domain authentication. Protocols like SPF, DKIM, and DMARC validate your sender identity and signal trustworthiness to recipient servers. Without proper authentication, your emails may be blocked.

Of course, you may not have received any notice that your email won’t send.

Instead, you may be missing replies to critical emails, or have received reports from customers that they’re not receiving your emails. 

This is where email deliverability comes in. Is it not sent or not in the inbox?

Why won't my email send: Deliverability Issues

Deliverability extends beyond just sending an email to ensuring your message lands in the recipient’s primary inbox rather than the spam folder. 

Poor email deliverability can impact all of your company’s emails, from prospecting email sequences to important transactional emails which are required to use your services seamlessly. As an examples, here are the types of emails which will start getting filtered into spam (and lost) if you don't proactively monitor and improve email deliverability:

• Customer Onboarding Emails - Meaning you see increased complaints to support issues.

• Billing Emails - This causes challenges for your accounts team trying to collect invoice payment.
  
  
• Customer Marketing Emails - Meaning it's harder to engage your user base and build brand loyalty.

• Bottom of Funnel Sales Emails - This means even the last few emails required to negotiate and get a customer contract signed could be lost.
  

Therefore, you'll want to consider the below key factors that can affect email deliverability, and overall success.

Sender Reputation

ESPs use sender reputation to determine how trustworthy your domain is. If you have a low sender reputation, there’s a good chance your emails are headed to spam.

If you have a low sender reputation, your domain likely has a history of receiving a high number of bounces and spam complaints.

Equally, if you send to invalid, harmful email addresses or even spam traps, this can result in your emails being marked as spam or blocked entirely.

Engagement

User engagement has become one of the most critical metrics ESPs use to evaluate email relevance and deliverability.

• Positive Engagement actions like clicking through email, replying, or marking messages as “not spam” indicate that recipients find your content valuable. 

These signals increase your chances of reaching the inbox.

• Negative Engagement: If users are deleting emails without reading them, marking them as spam, or consistently ignoring them, this sends red flags to ESPs. 

Low engagement rates can lead to your emails being deprioritized or directed to spam folders.

Content

Spam filters look for certain triggers like excessive links, spammy phrases, or poorly formatted emails. Additionally, content relevance is key—short, tailored messages that resonate with recipients tend to perform better.

Sender reputation and engagement metrics often outweigh all other factors. Even with well-crafted, properly authenticated emails, poor reputation or low engagement can prevent delivery. 

To optimize deliverability, focus on building a clean email list, warming up and authenticating your domain, and consistently creating content that resonates with your audience.

Let’s dive a little deeper into how to improve your deliverability and how to make sure your emails are not only sent but also get sent to the right email folder.

How to ensure your emails send

1. Email List Hygiene

Maintaining a high-quality contact list is essential. Start by sourcing data from reputable providers to ensure your contacts are valid and aligned with your target audience. 

Regularly clean your lists to remove invalid emails, frequent spam reporters, and low-engagement users–you should do this at least once a month. This ongoing risk management practice is essential for optimizing deliverability and protecting your sender reputation.

Beyond validating email addresses, evaluate contacts for the likelihood of marking emails as spam or ignoring content. Tools like Allegrow Safety Net Everywhere can streamline this process, helping maintain list health.

Here's a brief overview video of how Allegrow does this automatically:
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2. Domain Health

Email providers prioritize domain reputation when assessing a sender's trustworthiness. Domain reputation depends on recipient engagement, with high engagement boosting reputation and low engagement harming deliverability.

Before sending large email volumes, it’s important to make sure authentication protocols are correctly set up and that you warm up your domain by gradually increasing email volume over weeks. 

Using a subdomain for email outreach also protects your main domain’s reputation, ensuring critical communications aren’t impacted if deliverability issues arise. 

3. Optimized Content

Generic email content doesn’t resonate well with recipients; personalization is key to fostering meaningful connections in cold outreach. Hyper-personalize emails by referencing social media engagement, online reviews, job postings, tech stack changes, or insights from review platforms are great ways to engage users.

It’s important to also avoid large attachments and excessive links, as these can trigger ESP security checks or spam filters. 

Subject lines can also significantly impact deliverability and engagement. Spammy or emotive subject lines may trigger filters, while clear, straightforward lines perform better. Testing shows shorter subject lines—3 to 5 words—often achieve higher engagement, especially when they’re simple and direct.

Future Email Sending Success

Email remains a vital communication tool, but technical errors and deliverability challenges can disrupt its effectiveness. By addressing these issues and implementing best practices, you can ensure your emails are not only sent but also land in the primary inbox.

Allegrow’s advanced email deliverability tools can help you monitor, optimize, and improve email sending. Whether you’re troubleshooting sending issues or aiming to boost your deliverability, Allegrow has the expertise to keep your emails on track.

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