Mapping drives to server outside of domain on a different subnet

Are you supplying alternate credentials? Think you need to use this option to make this work

On the Map Network drive window select 'Use Alternate credentials' and enter the remote domain\user + a password and you should get connected

Are you able to browse to the share wuthout mapping it?    If not then you may have a dns issue

I am supplying alternate credentials. I cannot browse the share without mapping it.
This is only from Server 2008R2 machines. Windows 7 clients and Server 2003 machines can browse and map to the share.

Are all the machines using the same DNS settings?
If the client Win 7 machine can connect verify that the DNS settings its using are the same as the server firstly...

When you map the drive from the Win7 machine you are using alternate creds yes? Or does it map direct?

All clients are using the same DNS Servers. I can successfully perform NSLookup from any server or client. However I am unable to ping the server I am trying to map to from any 2008R2 servers. I can ping it from all other servers and clients.

Ok, you're trying to access, drive map,  a win2003 server (server a) from several win2008 r2 servers (server group a) which fails.. you are able to access, drive map, server a from several win2003 servers (server group b)which also reside elsewhere..

you are able to ping server a from server group b but cannot ping server a from server group a
 
ok, do the 2008r2 servers need a specific routing towards this destination server?
do the 2008r2 server have windows firewall enabled on them?

is there a specific access rule on a eg. cisco firewall required to access this destination server? This might already be in place for the 2003 servers and win 7 clients.

could you try creating the same local user on a win2008 server and on the destination server eg. test1 with the same password and using this user to drive map?

let me know if this works

I try these suggestions tomorrow. I did find, from the server I cannot map to, I can ping everything execpt the one 2008 Server I am trying to map a drive from.

Ok, well if the arp clearing cache thing doesn't work then a switch reboot may be the next thing to clear its cache

But we'd need to know what's in between this server and the destination server(are we on the same LAN segment, I know you mentioned different domains?)

If there's only 1 switch then its a simple thing, if we are crossing routers/firewalls(as @zackmccracken says) then whole different issue...sure let us know either way thanks...

Arp -d *
On both servers and retry

This fixed the problem.
Thanks!