Installing Dovecot after Postfix is installed.

That's because there is no package called "dovecot-postfix" (and you forgot "install"). You can just "sudo apt-get install postfix" now and "sudo apt-get install dovecot" later. "postfix-dovecot" is just a virtual package that provides both.

Thank you squircle that was very helpful.

Apparently the system is out of date. When I issue the command:

apt-get install dovecot

it tells me that "dovecot" is no longer available; however, it did tell me to use "dovecot-common" instead.

So I typed

apt-get install dovecot-common

it worked.