I separate personal and work emails by app I use. I have my work email configured in apple mail app and personal in one of the webmails. Recently a new colleague told me that I regularly send him emails to his personal email. Of course, because I was communicating with him during hiring and his personal email is alphabetically first.

First I went to contacts app and deleted his personal address. But it didn’t work.

The solution: In Apple Mail select menu window/previous recipients and delete unwanted email.