User anlegen

Ganz einfach im Terminal adduser und den Namen eingeben und danach das Passwd für den Account hinterlegen

adduser [name]

Also:
root@postfix:/home/jj# adduser nextcloud
info: Adding user `nextcloud' ...
info: Selecting UID/GID from range 1000 to 59999 ...
info: Adding new group `nextcloud' (1006) ...
info: Adding new user `nextcloud' (1006) with group `nextcloud (1006)' ...
info: Creating home directory `/home/nextcloud' ...
info: Copying files from `/etc/skel' ...
New password:
Retype new password:
passwd: password updated successfully
Changing the user information for nextcloud
Enter the new value, or press ENTER for the default
        Full Name []:
        Room Number []:
        Work Phone []:
        Home Phone []:
        Other []:
Is the information correct? [Y/n]
info: Adding new user `nextcloud' to supplemental / extra groups `users' ...
info: Adding user `nextcloud' to group `users' ...
root@postfix:/home/jj# 

Danach den Postfix neu einlesen lassen, bin mir nicht sicher ob das wirklich notwendig ist, habe ich noch nicht ausgiebig getestet.

Korrigiere, ist nicht notwendig, geht auch so.....

systemctl reload postfix