When I installed Microsoft Office 2007 and started to use Outlook I was surprised to find out, that I can enter only one outgoing (smtp) server per account. Using two different ISPs (and three accounts), one at home and one at work, required changing outgoing server and this started to annoy me.
There are good news for all of you with the same problem. There are at least three ways to solve it.
1. Get a web e-mail account that provides pop/smtp access and set that smtp server as outgoing server for your account(s).
2. Create duplicates of account(s) - one copy of account for each provider in your current profile (that makes 6 accounts in my case). Also, you should set appropriate account properties (outgoing server, incoming folder, other sending options). Otherwise it can happen that you receive messages twice etc. Each time you send a message you have to select correct account copy for current ISP. I admit, this is not the most elegant solution.
3. Create new profile.
For Windows XP open Control Panel -> User Accounts->Mail
Select Show Profiles and create new profile with the wizard. Wizard will create new .pst file (usually new file is named Outlook1.pst) for this new profile. However, if you wish (I did!) you can change this by selecting newly created profile from the list of profiles and clicking Options. Click on Data Files... and select the original Outlook.pst file.
Now both of your profiles will use the same .pst file. This means you will use the same Inbox (Calendar, Contacts, Tasks, etc.) for both of your profiles and that all of your things will be in one place.
After this, restart Outlook, select appropriate profile, create all of your accounts (this is the ugliest part) and set the outgoing server of other ISP for all of them. This solution works best for me. Unfortunately, you would have to create new profile for each new ISP and re-create account(s) for each new profile.