Change the default SMTP port to `25`, to better match the default
address `localhost`. This gets rid of some error outputs in the test,
where it fails to connect to localhost:465.
Also, don't enable postfix by default unless it's actually useful to
us.
The NixOS 21.03 release has been delayed to 21.05. See NixOS/rfcs#80.
There are two instances of 21.03 which have been left as is, since they
are in stateVersion comparisons. This will ensure that existing user
configurations which refer to 21.03 will continue to work.
