Make sure your key size is big enough by editing (I suggest 4096):

vi /usr/local/etc/courier-imap/imapd.cnf
vi /usr/local/etc/courier-imap/pop3d.cnf

cd /usr/local/share/courier-imap
mv imapd.pem imapd.pem.old
mv pop3d.pem pop3d.pem.old
mkimapdcert
mkpop3dcert
/usr/local/etc/rc.d/courier-imap-imapd-ssl restart
/usr/local/etc/rc.d/courier-imap-pop3d-ssl restart