I have several email accounts configured in my mail app.
For no reason it started sending email from one of my accounts with another email address, so when people reply to my email (from a particular account) it goes to another inbox and I sometimes miss it.
MacBook Pro (15-inch Early 2011), Mac OS X (10.7.5)
In Mail>Preferences>Accounts>you can set Each Mail account to "Use Only This Server" located at the lower right of the window below, which you specify once. Then the Mail app will not have a choice of server available to pull those shenanigans.