How can I change the return address ?

Question

I have an email forwarding service with it's own email address. Any email sent there will forward the email to any address I specify. I want to be able to write messages from my Mail account and have the return address (i.e. when they reply it goes to the email forwarding service and will then be forwarded to my Mail address) be the one I use at the email forwarding service.
I have tried changing the email address in Accounts/preferences/Account info to be my other email address but Mac email address and password is needed in those 2 fields to send mail . There is no other place (as far as I can see) to put my alternate email address.

Answer 1

Whether the following process works depends on whether the outgoing server you normally use allows you to do this - some do, some don't.
Create a new account in Mail Preferences>Accounts, The Description can be anything convenient, the Email Address is that which you want to use, and enter your name. For Incoming Mail Server, Usere Name and Password enter any old garbage (If you do this with more than one address the User Name garbage must be different for each of these accounts you set up).
Set up the Outgoing Mail Server the same as the one you normally use.
Now click on the 'Advanced' tab. Check 'Enable this account' but do not check 'Include when automatically checking for new messages' (or you will keep getting error messages on auto-checking. Note that if you manually 'Get all new mail' you will get error messages for these accounts).
Now the domain addresses will appear as options in the 'From' drop-down menu beneath the Subject field.
In Mail Preferences>Composing set 'Send new messages from' to 'Account of selected mailbox' and you should find that when replying the 'From' address matches the one the message was sent to.