How to fix “IMAP or SMTP port error” on Proton Mail Bridge

In this guide, we show you how to fix an [SMTP or IMAP] port error on Proton Mail Bridge.

Why am I seeing this error?

Ports are the virtual endpoints of a network connection. All kinds of networked programs use the ports on your device to connect with the internet, including Proton Mail Bridge.

Bridge uses ports 1143 and 1025 by default. But sometimes, one or both of these ports are already in use by another application, such as voice over IP, streaming media apps, another email service, etc. If that happens, you will see a port is already occupied error.

The error message may appear before you configure your email client or at any point afterward if another app begins using the Bridge ports.

The first step to resolving this issue is to change the port numbers used by Bridge.

How to change port numbers in Bridge

To assign Bridge new port numbers, go to Settings → Advanced settings → Default ports → Change.

Increase the relevant port number by one (for example, to 1144 for IMAP or 1026 for SMTP) and click Save.

If you see the error again, continue to increase the port number incrementally until you no longer see the error.

Updating port numbers in client settings

If you haven’t yet configured your email client, follow the instructions for your client(neues Fenster). The client configuration will automatically identify the ports Bridge is using. There’s nothing more you need to do.

However, if you’ve already configured your client, you’ll need to update the ports in your client settings. This process is slightly different for each client.

Thunderbird IMAP

Open the Account Settings tab, click on your Proton Mail account, and go to Server settings → Server Type: IMAP Mail Server → Port and change the port number to match the IMAP port number in Proton Mail Bridge.

Thunderbird SMTP

Open the Account Settings tab, click on your Proton Mail account → Edit SMTP server…

Change the port number to match the SMTP port number in Proton Mail Bridge.

Outlook for Windows

Newer versions of Outlook

Depending on the version of Outlook you’re running, go to File → Account Settings → Sever Settings or File → Account Settings → Account Settings.

Select Incoming mail to change IMAP settings or Outgoing Mail to change SMTP settings. Change the Port number to match the IMAP or SMTP port number in Proton Mail Bridge.

Older versions of Outlook

If you’re using an older version of Outlook and the above option isn’t available to you, then:

1. Click Info → Account settings.

2. Select your email address and click Change.

3. Click More Settings.

4. Click on the Advanced tab and change the Incoming server (IMAP) or Outgoing server (SMTP) number to match the IMAP or SMTP port number in Proton Mail Bridge.

Outlook for macOS

1. Open Outlook and go to the menu bar → Tools → Accounts…

Select your Proton Mail account and go to Incoming server (for IMAP) or Outgoing server (for SMTP).

Ensure Override default port is checked and change the relevant port number to match the IMAP or SMTP port number in Proton Mail Bridge.

Apple Mail

1. Delete your Proton Mail account. To do this, open the macOS System Settings, search for Profiles, and select Install, view, or remove configuration profiles.

Select the account you wish to remove and click – (and confirm when asked).

2. Set up Proton Mail Bridge on Apple Mail again.