First figure out the IP address of both your work's SMTP server, and home's SMTP server that you normally use to access them.
Check in Outlook and set it to use you work's SMTP server by name.
\windows\system32\drivers\etc, and make a copy of your HOSTS file (name it
HOSTS.2 or something).
Edit the first HOSTS file, and add and entry for the SMTP server's name (from Outlook), with your work's IP address for that server.
Edit the other to add the same Server name entry, but put your ISP's SMTP IP in instead.
You can then create a batch file to rename/swap the files, something like:
ren HOSTS. HOSTS.bak
ren HOSTS.2 HOSTS.
ren HOSTS.bak HOSTS.2
Then you can just run the batch and it will swap HOSTS files, which will cause the server name to point to whichever IP is in the active HOSTS file.
If you are running Windows 7 you can go further by creating two batch files (one to make the home HOSTS active, and one to make the work HOSTS active) and then create two Scheduled Tasks that are set to trigger on login (or alike), but only if connected to a specific network:
Then, when you log in it will check to see which network you are attached to and run the proepr HOSTS rename batch file for that network.