is a self signed certificate always invalid the first time?

So i’m using dovecot, and i created a self signed certificate with mkcert.sh based on dovecot-openssl.cnf. The name in there matches my mail server.

The first time it connects in mac mail however, it says the certificate is invalid and another server might pretend to be me etc.

I then have the option of trusting it.

Is this normal behaviour? Will it always be invalid if it’s not signed by a third party?

Thank you.