Originally Posted by Loren Pechtel
Something to keep in mind: It very well might be that you have no choice in the matter, that you must send your e-mail through your ISP's account. Many ISP's are blocking outgoing e-mail unless it's being sent through their server. The thing is most e-mail being sent from home users and not through the ISP is due to malware.
Any decent email program will allow you to set up multiple outgoing (smtp) accounts. In the preferences/options for each account, you chose the outgoing server for that email account.
If you mean that spoofing the return address won't necessarily work, I agree.
-David