You can have 2 default servers and you can also have no default servers, although I am not sure how to reproduce having no default servers. Somehow you can end up with no Is Default being checked. Tried everything to reproduce it.

Here is how to reproduce the NO Default server situation.

Adding a second server, then deleting it, will show that you have one remaining server and that it is default. However, next time you open the SMTP configuration, you will see that no server is a default server.