Can I change the email address notifications come from?

Can I change the email address notifications come from?

I'm setting up a site for a client, and I'm the admin, but the notifications need to come from a clients address (the service provider).

Though it would be nice to be able to assign a user as the "Appointments Manager" or something to determine where the emails come from, I'll take some function.php code to override the address if that's possible.

Thanks,
Michael