How to create Maildir directory with enough permissions for Postfix?

Edit: answer completely rewritten according to comments

The issue could be related to SELinux. You can run e.g. sestatus to check if it is enabled or disabled.

For maildir delivery, postfix changes to the corresponding user, so the destination directory needs to be writable by the user. This seems to be already the case. For privacy reason, I suggest chmod -R o-rwx /var/spool/mail/*

Just for completeness: If mbox files are used, the spool directory needs to be writable by the mail group which you get by using chmod -R g+rwX /var/spool/mail.