Cannot write to cifs share with SMB/Samba between 2 linux machines

As user1 (primary group root) on server1 (Linux) I am able to write files to /tmp.

The permissions are:

drwxrwxrwt  19 root              root  4096 Jan  3 11:04 ./

When I mount (cifs) server1 using usern=user1 on server2 (Linux) I am unable to write any files.

user2@server2$ echo hello >> /mnt/server1/tmp/foo
-bash: /mnt/server2/tmp/foo: Permission denied

However, I am able to write files from a Windows client when I connect with user1!

/etc/samba/smb.conf on server1

[root]
        comment = Root
        available = yes
        path = /
        write list = root user1 @root
        read only = no
        browsable = yes
        public = yes
        writable = yes

/etc/fstab on server2

//server1/root    /mnt/server1   cifs    user=user1,password=***,dir_mode=0770,file_mode=0770,nounix    ,uid=0,gid=0,forceuid,forcegid

I have tried with/without the uid=0,gid=0,forceuid,forcegid options as well as the mode and nounix options.

From server2I can list files in /mnt/server1/tmp and see files I created locally on server1. I don't see any permissions errors in the logs /var/log/*.

Where/how can I diagnose the issue?

UPDATE

I am able to write to the share from server2 as root. This is really strange as I would think server1 would not trust that root from server2 is the same root on server1.