There are two ways to deal with WordPress Multisite on Directadmin powered server, and you should choose one according to your preferences and requirements.
First of all you should ask yourself a question:
Please answer the question now and it will help to determine a way which you will go further while setting domain mapping for WordPress Multisite in the control panel for web hosting Directadmin.
If you need separate email boxes per domain, then you should never use aliases (pointers) when adding a new domain in Directadmin for WordPress Multisite, and that is why.
Directadmin shares one password file for all mail accounts between domains which are added as aliases.
It means that if you have example.com, example.nl, example.eu added as aliases, then if you create firstname.lastname@example.org, you will get aliases with names email@example.com, firstname.lastname@example.org as well (with a single disk storage and a single password).
Thus if you are planning to give access to read email@example.com, firstname.lastname@example.org or email@example.com to different users then you should add every new site in directadmin as a regular domain with its own public_html and private_html. On the moment of creation public_html can be replaced with a symlink, so that you will still be able to use WordPress Multisite and have independent mail boxes.
There are other moments which you should consider too (probably more technical and less common):
Of course you can mix it and add some domains as aliases if you don't want to have separate logs, stats, mailboxes, settings for it, and add other ones as independent domains if you want to separate them for any mentioned reasons.
Please contact us if you need our help regarding the matter and/or automation of the process.