Linux – Unable to access samba server

I can't connect to my samba server running CentOS 6.4 (minimal installation) that is not located within my network.

When I try to connect with my Windows 7 client, the connection fails and I get the default "unable to access " dialog displayed.

What I've tried so far:

• Opened ports 137, 138, 139, 445 on my samba server
• Verified that the ports are open by using nmap (on the samba server)
• Tried to telnet on the mentioned ports above from my client but the connection also failed
• Reinstalled samba and reconfigured the service
• turned off SELinux temporary

iptables

# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:https
ACCEPT     udp  --  anywhere             anywhere            state NEW udp dpt:netbios-ns
ACCEPT     udp  --  anywhere             anywhere            state NEW udp dpt:netbios-dgm
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:netbios-ssn
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:microsoft-ds
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited

nmap

#nmap -sTU -O localhost

Starting Nmap 5.51 ( http://nmap.org ) at 2014-05-16 17:29 CEST
Nmap scan report for localhost (127.0.0.1)
Host is up (0.000042s latency).
Other addresses for localhost (not scanned): 127.0.0.1
Not shown: 1994 closed ports
PORT    STATE         SERVICE
22/tcp  open          ssh
25/tcp  open          smtp
80/tcp  open          http
139/tcp open          netbios-ssn
445/tcp open          microsoft-ds
68/udp  open|filtered dhcpc

Samba version

#smbd --version
Version 3.6.9-168.el6_5

workgroup is set up correctly in the smb.conf, nmb is running.