I cannot delete postmaster@example.com or admin@example.net. The "delete" button is grayed out. Explanation:

I am new to Zimbra, we have a small multi-tenant setup (hosting provider). We have Network Edition with licenses but are closing the license limit and would need to remove some unnecessary accounts.
- admin@example.com
- postmaster@example.com - I want to delete this, do not know if it was automatically generated or not. I know the function of postmaster so if it simply cannot be deleted I want to consolidate admin@example.com into it.
- admin@example.net - I was stupid and create an separate admin account for a client instead of delegating the rights to correct account, now I delegated the rights and want to delete this.

The config is like:
-server name: mx1.example.com
-initial domain: example.com
-customer1 domain: example.net

How to get rid of the two unnecessary accounts? At least admin@example.net should be possible to delete, not a special system account??
And how to consolidate the postmaster@primarydomain and admin@primarydomain accounts??