How to resolve issues with “Checking mail server capabilities” notice in Thunderbird

When you use Thunderbird with Proton Mail Bridge, you may encounter this status notification.

Typically, this remains for a few seconds and then disappears. If it remains for more than two minutes, this might be due to a configuration issue.

What causes this issue and how to resolve it

 By default, Thunderbird uses a secure connection to communicate with Bridge. If there are any inconsistencies between the Connection Security settings in Thunderbird and Bridge, this notice may appear.

Follow the steps below to resolve it..

1. Ensure connection security settings match in Thunderbird and Bridge

First, open Bridge and choose your account. You will see connection security settings on the right side for both IMAP and SMTP.

In Thunderbird, click on Menu at the top-right → Account Settings and choose your Proton Mail account from the left side. Select Server Settings, and ensure the connection security matches that used by Bridge.

After making the changes, restart the email client.

If connection security settings already match, you can move straight to the next step.

2. Re-approve the certificate in Thunderbird

The certificate in Thunderbird needs to be manually trusted. If you have done this with an older certificate (before resetting Bridge, for example), Thunderbird may now have the incorrect certificate. In that case, you’ll need to remove the old certificate from Thunderbird. 

To do this, open Menu at the top-right → Settings → Privacy & Security→ Manage Certificates → Servers tab, and remove all entries for 127.0.0.1.

Restart Thunderbird and click Get Messages, Thunderbird will ask you to trust the certificate.

Make sure Permanently store this exception is checked, and then click Confirm Security Exception.

The next time you send a message, you’ll be asked to repeat this process for the SMTP server as well.

3. Switch connection security to “None” in Thunderbird

If the second option didn’t work, you can set connection security to None, which will bypass certificate checks.

This will only impact the connection from Bridge to Thunderbird which does not leave your device. The connection from Bridge to Proton servers is always secured with TLS. As this connection never leaves your device, this option does not pose an additional risk as long as your device is secure.

First, open the Bridge, and navigate to Settings → Advanced Settings → Connection mode. Ensure both IMAP and SMTP connections are set to STARTTLS.

From Thunderbird, open the Menu at the top-right, and choose Account Settings. Select your account and choose Server Settings. Change Connection Security to None.

Select your account once again. At the bottom of the page you’ll see Edit SMTP Server. From the pop-up, change the connection security to None.

This will allow Thunderbird to connect to Bridge without requiring a certificate.