Why does mailutils no longer contain Postfix on Ubuntu 20.04?

ubuntu postfix ubuntu-20.04

Ubuntu hasn't changed at all. It will still install postfix as a dependency of mailutils, if you don't already have some other mail transport agent installed.

The package lists as its dependencies:

default-mta
virtual package provided by postfix
or mail-transport-agent
virtual package provided by [show 15 providing packages]

Because you have replaced postfix with one of the listed packages, it would not be installed. If you really want postfix, you will need to remove the other MTA before installing postfix.

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