Kaspersky Internet Security 2017 cautions that the email anti-virus is not compatible clients that use "UI Design".

What in the world does that mean? What is "UI Design", and how is that related to anti-virus or Thunderbird in the first place?

In any case, letting anti-virus software scan incoming and outgoing messages is a bad idea, so it's best to disable that in your Kaspersky settings.

I don't know what "UI Design" is. I've seen references to it on forums like this and can only assume that it is a new approach to email client design. Can I assume from your response that Thunderbird doesn't use it? It's hard for me to see how not scanning incoming email for viruses can be a good idea. Thanks.

Can I assume from your response that Thunderbird doesn't use it?

I don't know. You should ask Kaspersky what their concern is about "UI Design". I don't see how this is relevant in the first place.

It's hard for me to see how not scanning incoming email for viruses can be a good idea.

This forum is full of complaints where anti-virus did screw up Thunderbird mail files resulting in lost messages. Technically there is absolutely no need to scan email messages and Thunderbird mail files. You can do your own research. This article may be a good start. http://thundercloud.net/infoave/tutorials/email-scanning/

I'd be curious why you believe scanning incoming email for viruses is a good idea.