Hi I have just spent many hours figuring out the answer to this problem so I thought I would share the answer.
I am running evolution 3.28.5-0ubuntu0.18.04.2 on Linux Mint 19.3 Cinnamon kernel 5.4.0-146-genericTLS.
I was having some problems with my TLS mail certificates on my server, and, as part of the diagnosis, i went into evolution and made some changes to the mail certificates which had been stored.
For some unknown reason, it is not possible to modify all of the mail ca-certificates stored in evolution as you are supposed to be able to do from within the gui. (Not sure why that is but, it is not a problem of file permissions).
This means that, if you change or delete any of the stored mail certificates from within evolution, then, your mail handshaking protocol will subsequently error out giving you a tls certificate error.
I have discovered that these mail ca-certificates are stored in the following file:-
~/.local/share/evolution/camel-cert.db
Therefore in order to remove all the stored mail server certificates, it is enough to delete the camel-cert.db file.
When you start up evolution again, then, a new file will be created, and you will be re-prompted to accept the various ca-certificates which you have previously saved.
Hope this can save a few folks several hours it has taken me to find the storage location for these ca-certificates.