Mail wants to reply from a different address

I have a Smart Folder set up to collect all of my incoming email from a certain group of work associates. Lately, over the last couple of months or so, whenever I hit reply to any of the emails in this group, it wants to send the reply from my personal email account instead of my work email account, which is the address that the emails were sent to. I only have this problem when replying to anyone in this smart folder. If I reply to any other emails it always replies from the address which the email was sent to. Has anyone else had this problem, or know of a possible fix? I've had it set up this way for years but it's never been an issue until just recently.