How to fix macOS Mail app failing to send emails due to SMTP authentication errors with Microsoft 365

Summary

This guide helps you troubleshoot and resolve: How to fix macOS Mail app failing to send emails due to SMTP authentication errors with Microsoft 365. Follow the steps below to fix the issue.

Common Causes

If you are experiencing SMTP authentication errors when trying to send emails from the macOS Mail app using a Microsoft 365 account, it is usually due to an outdated password, an app-specific password requirement, or incorrect server settings. This guide will walk you through the steps to restore your outgoing mail functionality.

Quick Fix Steps

1. Verify your internet connection.
2. Check your Microsoft 365 password.
3. Remove and re-add your Microsoft 365 account in Mail.
4. Update macOS and the Mail app.

Detailed Instructions

1. Update your Microsoft 365 password

If you recently changed your Microsoft 365 password via a web browser, the macOS Mail app may still be attempting to authenticate with the old credentials.

1. Open the Mail app.
2. From the top menu bar, select Mail > Settings.
3. Click the Accounts tab.
4. Select your Microsoft 365 account from the list on the left.
5. Update the password in the account details section.
6. Close the Settings window and attempt to send a test email.

2. Remove and re-add the account

Re-adding the account forces macOS to refresh the authentication tokens and server settings for Microsoft 365.

1. Open Mail > Settings.
2. Click the Accounts tab.
3. Select your Microsoft 365 account and click the minus (-) button at the bottom of the list.
4. Confirm that you want to remove the account.
5. Click the plus (+) button to add a new account.
6. Select Microsoft Exchange or Other Mail Account and enter your full Microsoft 365 email address and password.
7. Follow the on-screen prompts to complete the setup.

3. Check SMTP server settings

If you are manually configuring the outgoing mail server, ensure the settings match Microsoft's requirements.

• SMTP Server: smtp.office365.com
• Port: 587
• Security: STARTTLS
• Authentication: OAuth2 or Password

To verify these settings, go to Mail > Settings > Accounts, select your account, and click Server Settings.

Troubleshooting

App-specific passwords

If you have Multi-Factor Authentication (MFA) enabled on your Microsoft 365 account, the standard password may not work in the Mail app. You may need to generate an app-specific password via your Microsoft account security settings or use the OAuth2 login method during account setup.

Keychain Access issues

Sometimes macOS stores corrupted credentials in the Keychain.

1. Open Keychain Access (found in Applications > Utilities).
2. Search for your email address or outlook.office365.com.
3. Delete any entries related to your Microsoft 365 mail account.
4. Restart the Mail app and re-enter your credentials when prompted.

If you have followed these steps and are still unable to send emails, please Open a Ticket with our support team.

Still Having Issues?