Mail replies from wrong email account

MBP, OS X 10.4.11, Mail v2.1.3.

I have more than one email account set up on Mail and the strangest thing happens and I can't find out how to stop it.

Here is the problem:

- I have 2 accounts, 'Account A' is for my partner & 'Account B' is for me.
- Someone sends an email to 'Account A' (my partner) and CCs 'Account B' (me) into the same eMail.
- If I click reply from 'Account B' it sends the eMail from 'Account A' which is my partner, when it should be from me.

I have worked out that this is because 'Account A' was originally sent the eMail in The 'To' field as opposed to the 'CC' field in composing an eMail.' I know that I can manually change the 'From' field every time when replying to an eMail but it becomes an nuisance. I forget to do it when quickly replying as I consider Mail to reply from the account that I'm reading the email from.

I have gone to the compose section in the preferences and there is an option for changing this when composing a new email but nothing when replying.
Can you PLEASE HELP ME!!

You and your partner really ought to be using seperate OS X user accounts.