If you want to change it permanently, you can do the following:

In the rsreportserver.config file (sql drive:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportServer):

5) Populate <SMTPServer>x.x.x.x</SMTPServer> with the IP of the machine where you just setup the SMTP server.

6) Populate <From>you@yourwebserver.com</From>

7) Change this one to false <SendEmailToUserAlias>False</SendEmailToUserAlias>

8) Lastly, make sure you setup the domain as a permitted host such as this:

<PermittedHosts>

<HostName>yourwebsite.com</HostName>

</PermittedHosts>

However, I was looking for a way to dynamically change the EMAIL FROM on each mail.... haven't discovered any yet!!!