I have Server 2003 running in a Virtual Box guest VM. The host is Ubuntu 14.04. I have a mail service running in the guest.
Logged into the guest and
telnet localhost 110
responds with
+OK POP3
The Virtual Box VM is setup with NAT and port forwarding for 110. I have made sure the NIC has a unique MAC address. I have checked that no other VMs are port forwarding 110. Ubuntu firewall is not on. I am able to see the host from other machines and can connect to the guest via Remote Desktop through port forwarding it just won't connect to 110 or 143
In the host (Ubuntu) I type
telnet localhost 110
and it returns
telnet: Unable to connect to remote host: Connection refused
Tried
sudo tcpdump -n icmp
and it didn't show anything being blocked
tried
sudo netstat -tnlp | grep :110
and it showed nothing listening on that port.
EDIT: This is different to the "duplicate" question in that the problem is with VirtualBox. It would seem that VB is not listening and forwarding those ports, however it is port forwarding for other ports.
ANSWER: I thought that if VB was refusing to forward port 110 (for whatever reason) then I should try forwarding some other port (1100) and then change the email client. I did that and that resolved the problem though it would have been better to get a proper answer as this was not a "duplicate" question.
