I'd like to see Postfix support included in CPanel. It's more security focused than is Exim, can be extended far easier than Exim, and less of its features need to involve recompiling--also useful given CPanel doesn't compile Exim. It's compatible with much of what Exim already uses for filters, email forwarding and the like, but also gives more flexibility with regards error handling. Also a bonus in my book is greylisting, which Exim needs hacked into it to work decently. If CPanel can support Courier alongside Dovecot, thought should be given to giving us the option of choosing between either Exim or Postfix.