I'm a Mac newbie using Apple Mail version 8.2 on Yosemite 10.10.3: I have 5 mail accounts and have noticed that mails sent from any account often get mixed up with sent mails from the same or different accounts. It is a complete mess, e.g. a sent mail can be appended to another sent mail's header from a different account, or, a sent mail can appear in the midst of a group of mails sent to an entirely different address. I have searched but not found anyone else who is reporting the same problem. Please help?
Open your Mail app preferences. Make sure you are using following setting.
This way your Sent email is always going to be from the currently selected email account.
Open one of the Sent mail folders. In there add the "From" to double check.