[Dovecot] SSL error is it due to a self certified certificate?

Hi I have set up dovecot with a self signed SSL certificate, created with the mkcert.sh script.

Now I see the following in the log:

pop3-login: SSL_accept() failed: error:14094416:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate unknown [206.124.112.22]

pop3-login: Disconnected: rip=206.124.112.22, lip=204.3.153.71, TLS handshake

Is this caused by my self certified certificates. Or is it another problem?

Thanks

Matthew