When logged in as User1@mydomain.com and an email comes into the alias sales@mydomain.com which is configured for user1 it appears in user1's inbox.
If they click reply it will by default get sent from User1@mydomain.com instead of Sales@mydomain.com.

How can we change it so that if the email came into the sales@mydomain.com alias any replies automatically go out from sales@mydomain.com?

I know you can change it manually before sending the email by clicking on the send from address. I want to know how to do it by default. :)\

Thanks,

Gaz