If your ISP allows you to send email without validating first then I don't see a reason why you can't use their smtp.
If not, then you could surely install a mail service... that is provided your ISP isn't blocking that port to keep you from running your own mail service. Many of them do that nowdays.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.