Apple Mail client

to add an (external) account to Apple mail on macOS select "Other Mail Account..."

name the account and enter the mail address (in this step the password is not needed)

use your Uni ID instead of "xx000"

this data is not enough to set up a connection - the extended configuration box will be displayed

here the data for the external account at the universities mail servers is used - use mail.kip.uni-heidelberg.de instead for the KIP mail account

use your Uni ID instead of "xx000"

now the set up is done and you can select which progs shall use the account