In the mean time I've removed squirrelmail and installed roundcube... which as far as I understand is just the webmail and shouldn't change the mail configuration...
the original user's email email@example.com
is sending and recieving in evolution... I tried to add an additional email 'firstname.lastname@example.org' in ISPConfig and it is not recieving email... from evolution, but I've got a wordpress site on the server with email@example.com
as contact email and it did receive email from the wordpress contact form to firstname.lastname@example.org
As with squirrelmail, roundcube accepts none of the login credentials of email addresses created in ISPConfig...?