Mozilla Thunderbird mail client fails to connect to the Gmail server

This article concerns:

• Kaspersky Internet Security
• Kaspersky Anti-Virus
• Kaspersky Total Security
• Kaspersky Security Cloud
• Kaspersky Small Office Security

Mozilla Thunderbird may not be able to establish an SSL connection to the Gmail server through the IMAP protocol if a Kaspersky Lab application is installed on the computer and the option Always scan encrypted connections is enabled in it. To enable the option, go to Settings → Additional → Network → Encrypted connections scanning. 

To connect Mozilla Thunderbird or another mail client with similar issues to the Gmail server, you need to import the Kaspersky application certificate to the mail client manually. For instructions, see the Mozilla support site.

To find the folder with the certificate, open Windows Explorer → Local disk С: → The "ProgramData" folder → The "Kaspersky Lab" folder → The "AVP[application_version_number]" folder → The "Data" folder → The "Cert" folder → The "1.cert" certificate file.
ProgramData is a hidden folder by default. To access it, enable the displaying of hidden files and folders.

This issue may also occur with other mail clients.

Useful references

How to enable access to hidden folders