I have setup a server with Samba as the PDC for a network domain.

I have postfix, squirrelmail and dovecot to serve email to the clients using MS Outlook or webmail.

I have iptables turned off.

This is an extract from my linux system log-

Mar 27 12:35:00 server1 smbd[20704]: client2 (10.0.0.3) connect to service profiles initially as user user1 (uid=0, gid=501) (pid 20704)
Mar 27 12:35:08 server1 smbd[20709]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20709]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20710]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20710]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20709]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20709]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20710]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20710]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20711]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20711]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20712]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593)
Mar 27 12:35:08 server1 smbd[20712]: Unknown packet in reply_sesssetup_and_X_spnego
Mar 27 12:35:08 server1 smbd[20711]: [2006/03/27 12:35:08, 1] smbd/sesssetup.c:reply_sesssetup_and_X_spnego(593) 0.0.0.3) closed connection to service profiles
Mar 27 12:35:13 server1 smbd[20704]: [2006/03/27 12:35:13, 1] smbd/service.c:make_connection_snum(642)
Mar 27 12:35:13 server1 smbd[20704]: client2 (10.0.0.3) connect to service netlogon initially as user user1 (uid=0, gid=501) (pid 20704)
Mar 27 12:35:13 server1 smbd[20704]: [2006/03/27 12:35:13, 0] smbd/service.c:make_connection(794)
Mar 27 12:35:13 server1 smbd[20704]: client2 (10.0.0.3) couldn't find service home
Mar 27 12:35:13 server1 smbd[20704]: [2006/03/27 12:35:13, 0] smbd/service.c:make_connection(794)
Mar 27 12:35:13 server1 smbd[20704]: client2 (10.0.0.3) couldn't find service home
Mar 27 12:36:44 server1 smbd[20704]: [2006/03/27 12:36:44, 1] smbd/service.c:make_connection_snum(642)
Mar 27 12:36:44 server1 smbd[20704]: client2 (10.0.0.3) connect to service whiznet initially as user whiznet (uid=0, gid=501) (pid 20704)
Mar 27 12:37:46 server1 smbd[20734]: [2006/03/27 12:37:46, 0] lib/util_sock.c:get_peer_addr(1150)
Mar 27 12:37:46 server1 smbd[20734]: getpeername failed. Error was Transport endpoint is not connected
Mar 27 12:37:46 server1 smbd[20734]: [2006/03/27 12:37:46, 0] lib/util_sock.c:write_socket_data(430)
Mar 27 12:37:46 server1 smbd[20734]: write_socket_data: write failure. Error = Connection reset by peer
Mar 27 12:37:46 server1 smbd[20734]: [2006/03/27 12:37:46, 0] lib/util_sock.c:write_socket(455)
Mar 27 12:37:46 server1 smbd[20734]: write_socket: Error writing 4 bytes to socket 24: ERRNO = Connection reset by peer
Mar 27 12:37:46 server1 smbd[20734]: [2006/03/27 12:37:46, 0] lib/util_sock.c:send_smb(647)
Mar 27 12:37:46 server1 smbd[20734]: Error writing 4 bytes to client. -1. (Connection reset by peer)
Mar 27 12:37:46 server1 smbd[20735]: [2006/03/27 12:37:46, 1] smbd/service.c:make_connection_snum(642)

Anyone have any idea what could cause these errors?

Just a guess, but I would investigate how your clients are connecting to the Samaba server. How are they authenticating mostly. You might also look at turning iptables back on and enabling the ports that you need. Just a guess but with having them turned off, it may default to not having the ports/sockets available for them to connect. This is just a guess though.