That error message clearly points at a connectivity problem. VPN services in China are not fool-proof. Getting one that just works is a never-ending task in itself. The Chinese government actively finds and implements ways of blocking the VPN services themselves.

I only have this issue with Thunderbird though, it works well with the other websites/apps, so there might be something I am missing from the Thunderbird settings.

Like I said for instance with Gmail app on the phone it goes smoothly. I'd still like to have access to Thunderbird though because it's way more comfortable :)

The gmail andriod app is a glorified web page, very similar to what you get if you log into the gmail web site.

Gmail have a very long history of refusing connection for locations that are not familiar to you. So moving from say Sydney to Chengdu will garner a refusal to connect and a security alert email. Likewise if your VPN is not consistent with it's drop off IP address then this becomes an ongoing issue.

Finally as Stans indicates, the Chinese government operates what is generally referred to as the great firewall of China and in doing so actively implements VPN blocking. Just as the block all access to Google services.

I would suggest you change your location in Google services to the same general location as the exit point in your VPN and see if that helps but it might just be that the firewall in China is blocking all access to the relevant SMTP port used by Google as they want all emails to be sent using Chinese services for obvious reasons. As google only offer a single port, Your vpn might offer some sort of port translation to avoid that. You would need to ask them, it is beyiond the scopt of this forum.

Thank you for your help !

Need the VPN install please

Need the VPN install please

I do not understand this comment in the context of this topic.