I use Apple Mail to deal with all my email. It works okay for me so no intention or desire to change to something else.
The problem I have is with message flags. All incoming mail from members of my local political party are flagged with the national party’s corporate colour. Great. Works for messages sent to me and those sent from HQ. What I can’t get right is putting the same flag on outgoing emails to members of the national party or local group. I don’t want all my outgoing email to be coloured of course as some of my correspondents are members/supporters of other parties.
It would be useful if there were someway for my replies to inherit the flag already given to the original message and to apply the colour when I write new emails to people whose own messages would be flagged.
Decades ago, when a Linux user, I did this sort of thing with procmail but its security flaws make it an undesirable option now.
I’m not sure about replies automatically inheriting the previous email’s flags. But if you go to Mailbox > New Smart Mailbox, there are so many filters you can choose and combine, that I reckon you have a good shot of at least being able to create a folder with only those outgoing messages you want.
(I recently used smart mailboxes myself to make my incoming mail notifications more selective. I created a smart mailbox with a few accounts that I wanted to receive notifications for, and set Mail to only show notifications from that smart mailbox, instead of from all my accounts. Now I don’t get notified when something comes into one of my spam email accounts. )