To set up and use different identities within Outlook, you must first set these up within Google mail. Once you have set up one or more identities within Google mail, these will then be available for you to use from within Outlook.
If you want a separate identify when, for example, responding from a shared help mailbox, then you have to confirm to Google that you have access to that address by responding to an email that Google sends to that address. Once you have replied from the address in question Google sets up the new identity for use within Outlook.
Setting up a new identity (new 'from' address)
To set up a new identity:
Sign in to Google email and click on the cog icon in the top, right-hand corner of the mail window.
Select 'Settings' or 'Mail Settings' (depending which option shows) and then select the 'Accounts' tab from the set of available tabs in the 'Settings' window.
Screenshot: set up a new identity from the 'Accounts' tab in Gmail's settings.
Under 'Send mail as' click 'Add another email address you own'.
Add the name or identity you want displayed in 'Name' and in 'Email address' enter the email address.
Click 'Next Step' followed by 'Send Verification' so that Gmail will send a verification message to your other email address to confirm that you own it. If another window appears asking about SMTP server set up, just leave the defaults and click 'Next Step'.
Locate the message from Gmail. Open it and and click the link contained in the message.
Back in the 'Accounts' tab in Google mail, select the appropriate option under 'When replying to a message'.
Sending from a different identity
To send from a different identity in Outlook 2010 (there will be slight differences if using Outlook 2007):
Open a new email message.
If you do not already have a 'From' button next to the 'Send' button, select the 'Options' tab in the 'Ribbon' and click on 'From' in 'Show Fields' to add a 'From' button to your draft email.
Click on the 'From' button and select 'Other E-mail Address...'
Screenshot: Choose the from address.
Type in the required email address in 'From...', choose which address you want to send from (there may only be one option) and click on 'OK'.
Please note: Outlook remembers the addresses that you add, so in future you just need to click on 'From' to select the address you want.