The best approach is probably to use an external config file. So, you would change your web.config to say:
<errorMail configSource="elmah-errorMail.config" />
And then each developer would have their own copy of that file which would contain:
<errorMail from="no-reply@mydomain.com" to="myemail@mydomain.com" subject="Project Error" smtpPort="0" async="false" />
And then of course you also have the production values on your production web server.
I still don't understand why you want emails sent out to the developer though. Isn't a yelllow screen of death enough of an alert :)