This one has me stumped:

I have several POP/SMTP email accounts set up in Outlook 2013. One of them is misbehaving. Whenever I receive emails sent to that account, if I reply to them, the "From" field is set to one of my other email addresses. I have checked that the "Reply e-mail" setting for this account is also the same as the email address for it. But that doesn't work. I've also tried making this account the "send from this account by default" account, but that still doesn't work. All my other accounts are working fine. I can receive email on them and reply to them and the reply will go out from the same email address that the message it is replying to came to. But not this one account. And I cannot find anything different about how its settings are set up than how I've done the other accounts.

I have had this problem randomly and intermittently for years, with prior versions of Outlook. But it is happening 100% consistently now with this one particular account. I'm trying to switch to this account as my primary email account, but cannot do it if it's going to always be responding from the wrong email address.