Personally I think the issue is a misunderstanding of what a VPN is doing that causes most of the issues. Fundamentally a VPN tries to hide your location by obfuscation of the IP address you use to connect to the internet.

Your ISP may well require you to connect to the internet using their connection service to access and send mail. So that process stops working with a VPN as you are no longer connected to the internet using their service.

I would guess your "corporate server" is outside of the VPN, or on its own VPN/Proxy, so as far as it is concerned you are not moving so you don't have issues.

For your calendar server, when you could have been all over the world in 10 minutes via VPN. This is something most providers are rather shy of. If your phone says you are in Columbia and your VPN says you are in Moscow, and twenty minutes later in Durban then there is a fair chance your calendar provider may think hey how can you be in two places at once. The guy has been hacked. Institute questionable connection protocols, like insist on a password next connection, or suspend the account until you reply to an email.

Fundamentally a VPN works in direct reverse to most of the protocols instituted by legitimate organizations like banks, and social media where they are sure then know where you are to protect you. It is not that the tech world hates VPNS. They really like to think they are dealing with you and not a hacker and your VPN usage makes you look like you are being hacked. One thing that might help is to enable DNS over HTTPS in Thunderbird. That will at least stabilize the DNS server used as each VPN probably redirects that as well.

Thanks for taking the time to respond. I use several popular client VPNs; ExpressVPN and NordVPN. My understanding is these VPN products tunnel directly from the client PC (me) through my network to a defined VPN server in various places around the world. All communication through tunnel, in either direction, is encrypted.

This allows me to stop off at a Starbucks coffee store and access their wifi network then connect to any external server with everything coming through the tunnel encrypted (such network where the tunnel is running through is the Starbucks wifi network. Connectivity from the VPN server will be as I direct, e.g. to my IMAP email server using encryption and port 993.

So, my calendar is client based but is also connected to the corporate Zimbra calendar so I can see both my email and calendar from my iPhone. Unusual things happen in my Thunderbird calendar, of which the described anomaly with my VPNs is one. There are others which have nothing to do with the VPN.

That's why I am wondering what changing my VPN from ExpressVPN to NordVPN has to do with my unresponsive calendar responses to an event alert but has no effect on my email uploading and downloading.