I'm completely new to c5, and site email for that matter, and I spent hours trying to get email working last night - with eventual success. I didn't find the solution anywhere so perhaps this will help someone - and me the next time I have to do this.

My host is Dreamhost and I did their one-click install of c5. My problem was getting email to send from the website - both via the forgotten pwd link for the sign-in and the default contact us form and confirmation email for a simple email list signup. Solution was to set up a fully hosted email for the site through dreamhost (this I am currently forwarding to another email just to be sure nothings is getting lost but I'm confident I can remove that now). Everything worked when I set the super admin email to be this new email address (I don't know if it could be a different admin user instead) and I updated the Send Mail Method from the dashboard settings to be SMTP with the following info:

Mail Server: mail.yourdomain.whatever
Username: full email address you set up (name@yourdomain.whatever)
Password: pwd for that email account
Encryption: TLS
Port (Leave blank for default): 587

I didn't need to change the config/site.php file, though I see this might still be useful, nor did I have to click any unusual boxes in dreamhost or c5. Now I can put different email addresses in the "Notify me by email when people submit this form" option for the Contact Us form and now those emails are being sent to notify several site admins when someone submits this form (at least I hope it's sending to all email addresses, seems to be). Some emails are forwarded to a gmail account but I'm not having any issues with that. Everything seems good so far... If I learn anything new I'll try to update this!