I hate it when I do a google search and can’t find what I need, so I’m mostly posting this so the next time I get this error, I can do a google search and find the post I made about this error in order to fix it. Yay internet.
Should the error “could not instantiate mail function” pop up when using Joomla and doing something that requires using php mail(), and you happen to be using a Windows server (2008, running IIS 7.5 in my particular case), there’s a good chance you (or I, as the case may be) restarted IIS at some point and your SMTP server never bothered to restart itself. If you’re like me, you had to set up SMTP in IIS 6 for some bizarre reason, so open up the IIS 6 Manager, check if your SMTP Virtual Server is running, and if not, start it!
To test if it’s running without filling out a form or whatever, either put together a sweet little php file that spits out “yay” if email sends or “nay” if not, or just try “Forgot Password” in Joomla and see if you get an email.