Stefan Monnier wrote: > Still, there is to me no good reason not to allow installing both exim > and postfix at the same time. I think it's just a tradeoff between how > often this could be useful and how much work it takes to tweak the > packages.
An MTA has to provide certain things, or else it is not an MTA. One of those is a daemon listening on port 25. Another is a /usr/sbin/sendmail command that accepts new mail. There is no requirement that the sendmail command use localhost:25 to get the mail in; it's perfectly acceptable for it to: 1. Do all the work itself, or 2. Use a socket to hand it off to the rest of the MTA, or 3. Send it off to another machine somewhere, even if it's destiny is to come back to this box among other things. So one package has to control both the port 25 daemon and the sendmail command. Debian policy is that, whenever possible, a package providing a daemon should start that daemon at install time. This is, perhaps, one of the more controversial policies, but it is in effect and has been for many years. Therefore, installing an MTA will get you both a port 25 daemon and a sendmail command, and remove any previous installed services. If you keep the config directories in place, you can switch between MTAs and feel reasonably confident that when you do that, via apt install, it will be started with the configuration that you already set up for it last time round. The exception are some shared files like /etc/aliases, where you should not expect a restored MTA to restore that config as well. Hence my suggestion of etckeeper. If one is trying to learn different MTAs, a few days or weeks with each is not out of the question, and the 10-45 seconds of apt install to change between them is reasonable. -dsr-
