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Troubleshooting Issues with Sending Mail

If you are having difficulty sending mail, please try these trouble-shooting steps:

1)  Verify that your firewall or antivirus program is not blocking Postbox.

2)  Double check that your Outgoing server (SMTP) settings correct and match your ISPs recommended settings.

When creating mail accounts, Postbox will attempt to automatically determine Outgoing server (SMTP) settings.  In some cases, it guesses the wrong values.

Go to Accounts -> Outgoing Server (SMTP)" (at the bottom of the left pane—you may need to scroll down), and double-check server name, port, and other settings.

3) You receive an error while sending that says your server does not support secure authentication (SMTP-AUTH).

Go to Accounts -> Outgoing Server (SMTP)" (at the bottom of the left pane—you may need to scroll down), select your SMTP Server and click the Edit button. Uncheck the option to Use Secure Authentication and then restart Postbox.

4) Double check your SMTP user name.

Ensure that there are no trailing spaces, as this can cause you to fail to authenticate.

5)  Try deleting the password associated with SMTP server in the password manager.

  • Postbox > Preferences... (macOS) or Options (Windows) > Privacy > Saved Passwords...
  • Select on the appropriate row (labeled something like smtp://<username>@<hostname>) under "Site"
  • Click "Remove"
  • Restart Postbox and try to send again

6) Try turning off signatures

If a signature contains an image pointing to a resource that cannot be accessed, the message will not send.

Go to Options or Preferences / Accounts / [your account] / Identity and set the signature to "None".  Try sending again, and if it works, please examine the signature's HTML to make sure that it is pointing to a accessible image on the web.

7) Disable IPv6

  1. Open the Config Editor.
    * (macOS) Postbox > Preferences > Advanced > General > Config Editor.
    * (Windows) Tools > Options > Advanced > General > Config Editor.
  2. Click on I'll be careful, I promise.
  3. In the search field, enter the following: network.dns.disableIPv6 
  4. Double-click the preference to set this value to true.
  5. Restart Postbox.