Any firewall running on Machine B?

Also could you try mounting with 'sec=ntlmv2'

yes definitely, machine-B is connected to the internet so it is firewalled, however the firewall doesn't stop any other lan machine from connecting to its share. It gets even weirder.... machine-A has no problem connecting to other shares on the lan. The incompatibility appears to be only with the other centos5 machine - machine-B. I'm beginning to think this is a software bug/incompatibility that is unsolvable....

So no go with mounting with 'sec=ntlmv2'? What about hardening, can you think of any specific hardening action that may be causing this issue?

sec=ntlmv2 = no go. I don't know what 'hardening' means in this context.

Possible to paste the output for the following share section from smb.conf:

Also, did you try: I have removed the password section from the command it will prompt when you hit enter.

And the thing I am most curious about, why not NFS when both boxes are Linux?

smb.confmount -t cifs //ip-machine-B/share /media/machine-B -o user=domain\my-userI've not used NFS before, I'm not familiar with it.

Wait a sec which password are you using? Are you using the normal user password or smb password? Things you have to make sure for authentication to work are:

1. The user account you are using exist on destination (samba hosting server).
2. The user account password has been set using smbpasswd <user>
3. You are using not the normal account password but the password that has been setup using smbpasswd.

Try this because I think you are using normal account password.

If that doesn't work we can figure out something, if you are open for NFS setup let us know and we will assist you with that setup.

I've been doing some deeper digging on this and discovered that on machine-A the command returns nothing. machine-A can ping machione-B and get a response but that's all. Surely this is where the problem lies. For some reason machine-A can't see any ports on machine-B.
On machine-A if I try to nmap other ip's on my LAN it works fine, it shows the ports up on the other ip's but from machine-B...nothing. I'm puzzled as to why this is.