# Error: 404 Not Found

Source: https://catalystgroup.gg/docs/email-delivery/error-404-not-found/
Updated: 2026-10-19 · By Duane Garner

The mail service could not find what CRM-NG SMTP asked for, usually a domain, region or mailbox. Check those settings against your account.

> **Shown by API mailers (most often Mailgun, SparkPost and Microsoft 365):** The test result or the error log shows `404 Not Found`, sometimes with a short reason such as "domain not found" or "resource not found", when CRM-NG SMTP sends through a service's web API.

 

This article is for site owners using an API mailer who get a 404 status. It is most common with **Mailgun**, **SparkPost** and the **365 / Outlook** mailer.

 

## What this error means

 

A 404 means the service received the request but the thing named in it does not exist on its side. With email APIs the "thing" is part of the address the request goes to: your sending domain, your account's region, or the mailbox you send as. The credentials may be perfectly fine.

 

## Common causes

 
- **Mailgun**: the **Domain Name** in CRM-NG SMTP does not match a domain in your Mailgun account. Common slips are using `example.com` when Mailgun has `mg.example.com`, or a typo.
- **Mailgun**: the **Region** is wrong. A domain created in the EU region cannot be found through the US endpoint, and the reverse.
- **SparkPost**: the **Region** setting does not match the account (US or EU).
- **365 / Outlook**: the **From Email** is not a mailbox in your Microsoft 365 tenant, or it is a group or alias that Microsoft Graph cannot send as. Graph reports the user as not found.
- The domain or sending identity was deleted from the provider after setup.

 

## How to fix

 
1. Check the domain at Mailgun
  Sign in at `https://app.mailgun.com/` and open the sending domains list (under **Send → Sending → Domains**). Copy the domain name exactly as listed. Note whether the account shows US or EU.
2. Check the region at SparkPost
  If you sign in at `app.eu.sparkpost.com` your account is EU; `app.sparkpost.com` is US.
3. Check the mailbox in Microsoft 365
  In the Microsoft 365 admin center, open **Users → Active users** and confirm the From address is a licensed user mailbox, or a shared mailbox, in the same tenant as your app registration.
4. Update CRM-NG SMTP
  Go to **CRM-NG SMTP → Settings → General**. For Mailgun, correct **Domain Name** and **Region**. For SparkPost, correct **Region**. For 365 / Outlook, correct **From Email** at the top of the page. Save.
5. Send a test
  Go to **CRM-NG SMTP → Tools → Email Test**, fill in **Send To** and press **Send Email**.

 

## Verify it worked

 

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." The provider's logs show the message sent from the expected domain or mailbox.

 

## Notes

 
- For Mailgun, the domain you send from must also match **From Email**, or recipients may see the mail marked as unverified. Use **Force From Email** to keep it consistent; see [Force From Email](https://catalystgroup.gg/docs/email-delivery/force-from-email/).
- For Microsoft 365, a newly created mailbox can take a little while before Graph can find it. Wait and test again.
- A 404 from any other mailer usually means a setting that names something on the provider's side is wrong. Compare every field with the provider's dashboard.

 

## Related

 
- [Mailgun mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-mailgun/)
- [SparkPost mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-sparkpost/)
- [Microsoft 365 mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-microsoft-365/)
- [Sender name and address](https://catalystgroup.gg/docs/email-delivery/sender-name-and-address/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)