Mail problem

To avoid separate computer logins, my wife and I share one computer user account, but we have separate email accounts in Mail. With my mailbox selected, when I reply-to, forward or create a new message, my wife’s name often (but not always) shows up in the "From" drop-down menu. If I fail to notice this, it appears to the recipient as if the email was sent by my wife. The problem is erratic, occurring maybe 30-40% of the time.

In Mail Preferences I’ve checked "Send New Messages From Account Of Selected Mailbox", so I don’t understand why this is happening. What can I do to make sure my name always shows up as the sender? Thanks very much for any ideas.

It is not difficult to change the address of sender in the "FROM" drop-down on the address/copy/info panel of a new e-mail page

Thanks, Greenman. I agree, it's easy to simply select my name in the drop-down menu, but it's just as easy to forget. I'm still hoping someone can shed light on whether there's anything I can do so my name is the default sender when I'm working within my own Mail account. Seems illogical to me that it doesn't do that.