I don't run dovecot on my Mac, but I do always set up Postfix, at the very least to have a fully functional smtp server on the local machine that can be used in a variety of ways. I also don't "upgrade" versions of OSX, but rather do a clean install. Since I've saved all of my config files for things like Postfix, it is typically a snap to get it running again on the new OSX. This was the case with El Capitan. My /etc/postfix files (main.cf, master.cf, and aliases) all dropped in the new installation without any issues. The main issue with El Capitan is that if you want the master process listening all the time, you need to modify /System/Library/LaunchDaemons/org.postfix.master.plist. On El Capitan, you will need to disable SIP long enough to do this, as it lives in a SIP-protected location. I did this right after installling 10.11.0, and the subsequent upgrades to .1 and .2 have not overwritten my changes to that file.