As far I know you can set From email address to any when calling a service such as SendMessage irrespective of the connected account. Although I have not tested it, I think you can change account and password by calling SetConfigurationTable on the ConnectionInfo table of your MailThing. Refer this article.
Thanks for the update. I had readout the mentioned article.I have the few following queries.
1.Can we achieve the SetConfigurationTable service via ThingWorx?
2. If yes means the configuration of mail thing chance dynamically or else it will only change the from mail id from SendMessage snippet.