Gmail Configuration – Setting Default Send Address to Match ‘To’ Address in Replies

There is not an explicit way of doing that, but it is a realy fantastic idea that you should sugest to the Gmail team.

http://mail.google.com/support/bin/static.py?page=suggestions.cs&issue=116880&bucket=16937

I work in the same way with both emails is Outlook and you just need to get into the habit of checking the "From" address before you click send. Enabling the "Undo" lab goes a long way to rectifying mistakes.

Sure, and it's pretty easy too:

1. Go to Gmail settings
2. Go to Accounts and Import Tab
3. Click on Send Mail from another address
4. Enter the Forward account address.
5. The email should send to that address which should forward back to your Gmail.
6. Complete the steps in the email.

When you send a new email you can now change the from address to your forward account.