The operation can't be performed on the default e-mail address policy.

When Editing the default Email address policy in Exchange 2010, I always get the error:

The operation can't be performed on the default e-mail address policy. 

(Full details in the bottom)

My workaround, is to copy the details, with ctrl-c, and paste it in notepad, and edit the command, so you'll only set the needed.
E.g. I here needed to add a  domain, and make it the reply domain. So I will run the following in Powershell (remember to run powershell as administrator):

set-EmailAddressPolicy -EnabledEmailAddressTemplates 'smtp:@ADdomain.local','SMTP:@newdomain.net' -Identity 'Default Policy'

The Full error code is, when adding a domain, and setting it as default:
Error:
The operation can't be performed on the default e-mail address policy.

Exchange Management Shell command attempted:
set-EmailAddressPolicy -ConditionalDepartment @() -ConditionalCompany @() -ConditionalStateOrProvince @() -ConditionalCustomAttribute1 @() -ConditionalCustomAttribute2 @() -ConditionalCustomAttribute3 @() -ConditionalCustomAttribute4 @() -ConditionalCustomAttribute5 @() -ConditionalCustomAttribute6 @() -ConditionalCustomAttribute7 @() -ConditionalCustomAttribute8 @() -ConditionalCustomAttribute9 @() -ConditionalCustomAttribute10 @() -ConditionalCustomAttribute11 @() -ConditionalCustomAttribute12 @() -ConditionalCustomAttribute13 @() -ConditionalCustomAttribute14 @() -ConditionalCustomAttribute15 @() -RecipientContainer $null -EnabledEmailAddressTemplates 'smtp:@ADdomain.local','SMTP:@Newdomain.net' -Name 'Default Policy' -Identity 'Default Policy'