It’s kinda late to report, since this feature is so common now that even corporates support it:
(https://office-watch.com/2022/email-plus-addresses-how-to-use-them/)
but KMail isn’t working with it. So there are identities and aliases. Logically identities are waste of too many settings to be considered as records per each alias, especially frivolous “plus” alias (there can be hundreds per user). This definitely should be defined as alias. Or not even that, just parse + sign or regexp (Thunderbird does it in settings). If You decide to define it manually it won’t really help in KMail. It doesn’t do much. All it should do is to appear in the “From” address combo box and or set “From” address to the recipient address in reply automatically. That ticket is already raised:
https://bugs.kde.org/show_bug.cgi?id=341941
and many duplicates or forum requests exist.
I wonder if we could catch up to this phenomenom.
Thanks for listening.