If you've recently upgraded to Red Hat Enterprise Linux 4 and are trying to generate a new self-signed certificate for the Dovecot IMAP server, you've probably found that simply typing "make dovecot.pem" doesn't work. What you have to do is generate one "by hand" using this command:

openssl req -new -x509 -newkey rsa:1024 -nodes -keyout mykey.pem -out \ mycert-pem

Thanks to the folks on the Dovecot mailing list for helping me with that. :)