You can add multiple smtp servers to mail, I have the same issue. Since I send most of my mail from home I have my ISP's smtp as the default and when am away I choose a different smtp. As long as you have set-up the smtp as secure by adding your user name and password you should be OK. Even setting that up with you ISP's details.
I have a separate e-mail server to the ISP I use and generally have not had an issue since I added my user name and password.
I hope this helps?