[CentOS] Ignorant question on SSL certs

I'm getting endless complaints about my dovecot cert,
/etc/pki/dovecot/certs/dovecot.pem
which I created years ago following the dovecot instructions.

Do I really have to use a separate cert and key for dovecot?
Can I not use the "standard" cert in /etc/pki/tls/certs (and key)
from CACert.org ?

-- 
Timothy Murphy  
gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin