Sender address rejected: not logged in

This error appears when an email client tries to send a message without properly authenticating to the outgoing mail server.

In most cases, it means that the application (such as Outlook, Thunderbird, or Apple Mail) is not correctly logged in to the SMTP server.

Common cause

The most frequent reason is incorrect or missing authentication details in the email client configuration.

This includes:

• missing username
• incorrect password
• SMTP authentication not enabled

How to fix the issue

1. Check mailbox credentials

Go to your email client settings and verify:

• full email address is used as the username
• correct mailbox password is entered
• SMTP authentication is enabled

Make sure you are using the password for the email mailbox itself, not the mybox account password. These are different credentials. If you forgot your password, you can find a guide HERE that includes instructions on how to change the password for an e-mail account.

2. Re-enter login details

Remove the saved password from your email client and re-enter it manually.

Then save the configuration and restart the application if needed.

3. Verify SMTP settings

Ensure that your outgoing mail configuration is correct:

• SMTP server: correct mail server address for mybox
• Port: standard secure port (usually 465 or 587 depending on configuration)
• Encryption: SSL/TLS enabled if required
• Authentication: required (must be enabled)

Practical implications

• Incoming mail may still work even if sending fails
• The issue is always related to authentication, not message content
• Reconfiguring the account usually resolves the problem

Summary

The “Sender address rejected: not logged in” error occurs when the email client is not properly authenticated on the SMTP server. Fixing it requires verifying the mailbox credentials, ensuring SMTP authentication is enabled, and re-entering the correct email password.