Applications :: Mail App Always Ask For Sending Account Address?

I'm having this problem for a while now and did not find any topics about it on the net. If it is handled some other place already, the link, or way to find it would be nice.

I'm using mail.app with 5 different accounts at work. Different accounts are supposed to be for different groups of receivers. When sending mails I unfortunately tend forget to check if the right sending address (account) is chosen. Mail automatically chooses the one from which the last mail was send. And so it ends up that people receive mails from the wrong address and are getting confused by that.