Subscription Emails TO: Field is greyed out in SSRS 2008

When you try to subscribe SSRS report you might not be able to select other email address to send to because To field is grayed out,

How to make TO filed editable?

This need to be changed in the Report Server Config file. To change this navigate to the report server folder for SQL server. Under it will be the config file.

Set <SendEmailToUserAlias> false</SendEmailToUserAlias> 

True=The e-mail address of the user creating the subscription is used. This is the default value.To field is hidden
False=Any e-mail address can be specified.To field is visible.

How to receive emails without allowing to change TO field?
This way is more secure so no one can send the reports to outsiders or to their personal emails.

You need to set DefaultHostName and this works with SendEmailToUserAlias

Set <SendEmailToUserAlias> true</SendEmailToUserAlias> 

<DefaultHostName>domain.company.com</DefaultHostName>
<PermittedHosts>
     <HostName>domain.company.com</HostName>
</PermittedHosts>