Troubleshoot email client warnings about invalid certificates

After installing Avast Antivirus, some third-party email clients, such as Mozilla Thunderbird, SeaMonkey, The Bat!, or Microsoft Outlook, may show that the mail server certificate is invalid when you send and receive emails.

Mail Shield in Avast Antivirus uses an advanced scanning method for incoming and outgoing emails over SSL/TLS secured connections. Mozilla Thunderbird, Sea Monkey, The Bat!, and Microsoft Outlook have their own certificate stores and may require extra configuration to avoid warnings about invalid mail server certificates.

For Mozilla Thunderbird, SeaMonkey, and The Bat!, follow the steps below to stop warnings about invalid mail server certificates. You must first export the Mail Shield certificate, then import it into your email client's certificate store.

For Microsoft Outlook, follow the steps in the Troubleshoot Microsoft Outlook errors section to reinstall Avast Antivirus.

Export the Mail Shield certificate

Open Avast Antivirus and go to ☰ Menu ▸ Settings.
Click Search in the top-right corner.
Type geek:area into the search box, then select Avast Geek from the search results.
Scroll to the Mail Shield section. Ensure the box next to Mail shield: Scan SSL connections is ticked, then click Perform next to Mail shield: Export certificate.
Select a location to save the exported Mail Shield certificate (for example, your Windows Desktop) and click Save.
Click OK in the Avast Information dialog.

The Mail Shield certificate is exported. Continue following the steps below to import the Mail Shield certificate into your mail client.

Each individual email account in your mail client needs to be configured as required by your mail provider. Refer to your mail client's and mail provider's documentation for instructions.

Import the Mail Shield certificate

In Mozilla Thunderbird, click ☰ Menu (the three lines) ▸ Settings.
Select Privacy & Security in the left panel.
Scroll to the Certificates section, then click Manage Certificates....
Select the Authorities tab, then click Import....
Locate the Mail Shield certificate you previously exported from Avast Antivirus, select the file, then click Open.
Tick the box next to Trust this CA to identify websites, then click OK.
When the Certificate Manager screen appears again, click the Servers tab.
Ensure that there is no entry for Avast Mail Scanner. If an entry exists, delete all certificates connected to Avast Mail Scanner in the Servers tab. Then, click OK to save the changes.

The Avast Mail Shield certificate is now imported to Mozilla Thunderbird.

In SeaMonkey, click Edit ▸ Preferences....
Click the arrow next to Privacy & Security to expand the section.
Select Certificates, then click Manage Certificates....
Select the Authorities tab, then click Import....
Locate the Mail Shield certificate you previously exported from Avast Antivirus, select the file, then click Open.
Tick the box next to Trust this CA to identify websites and click OK.
When the Certificate Manager screen appears again, click the Servers tab.
Ensure that there is no entry for Avast Mail Scanner. If an entry exists, delete all certificates connected to Avast Mail Scanner in the Servers tab. Then, click OK to close the Certificate Manager.
Click OK in the Preferences window to apply the changes.

The Avast Mail Shield certificate is now imported to SeaMonkey.

In The Bat!, click Tools and select Address Book.
Beside Trusted Root CA, click ⋮ (the vertical ellipses) and select New Contact from the context menu.
Type Avast Mail Shield in the Name field and click the green tick sign in the New Certificate panel above.
Ensure that Avast Mail Shield is selected and click the green Manage Certificates panel.
Click Import....
Locate the Mail Shield certificate you previously exported from Avast Antivirus, select the file, then click Open.
When the Mail Shield certificate appears in the Certificates tab, click OK to apply the changes.

The Avast Mail Shield certificate is now imported to The Bat!.

Troubleshoot Microsoft Outlook errors

If you are seeing certificate warnings or errors in Microsoft Outlook, we recommend you reinstall Avast Antivirus. For detailed instructions, follow the steps below:

Uninstall your Avast Premium Security or Avast Free Antivirus.
Restart your Windows device.
Before reinstalling Avast, open Microsoft Outlook and check if the issue persists.

If the problem still occurs, it is likely not caused by Avast. In this case, check your email account settings in Outlook and ensure they are configured correctly according to your email provider’s instructions.

If the problem does not occur, proceed to the next step and reinstall Avast Antivirus.
Reinstall Avast Premium Security or Avast Free Antivirus.
Restart your Windows device again.

Once Avast Antivirus is reinstalled, open Microsoft Outlook and check if the issue persists.

Avast Premium Security
Avast Free Antivirus

Windows

Updated on: 21/07/2025

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Troubleshooting email client warnings about invalid server certificates

Export the Mail Shield certificate

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