Postfix unable to deliver to user's Maildir

Solution 1:

In order for mail to be delivered, postfix must be able to enter the user's home directory, and that directory must contain a complete Maildir tree containing the subfolders tmp, cur and new, all with the correct permissions.

Since you've already checked that the Maildir exists, has the correct folders, and has the right permissions, what remains is checking whether postfix (and the users themselves) can enter the users' home directory.

In order for that to work, the permissions have to be right every step of the way. So, not only does /home/user need to have the right permissions - so does /home. It must be set either to 750, if it's owned by a group that the users are members of, or 755 if it's not.

You can verify this by simply su'ing to a user and try to enter their home directory.