To remove an outdated formaly used e-mail address from the list taken into account when Apple Mail does autocompletion of e-mail addressss, open the list of previous adressees from the “Windows” menu, select the address in question an remove it from the list.
Using different sender e-mail addresses (aka “aliases”) for one(!) e-mail account in Apple Mail is as easy as that: Just list all addresses in the “Address” field of the account configuration form separated by commas. That all …
defaults write com.apple.mail AlwaysShowDuplicates -bool true
defaults write com.apple.mail _AlwaysShowDuplicates -bool true
(1-to-1 copy of the reference below)
You should now see BOTH the flag icon color AND color name, but you can resize the column to hide the color name.
To enable the use of a PGP Key assigned to one address as a genric key for all recipients within that domain, import the Key into your keychain, identify its fingerprint and issue the command1):
defaults write org.gpgtools.common KeyMapping -dict-add '*@example.com' <fingerprint w/o spaces>
to view all defined mappings, issue the command:
defaults read org.gpgtools.common KeyMapping
to reset the list to a single entry, modifiy the command above to:
defaults write org.gpgtools.common KeyMapping -dict '*@example.com' <fingerprint w/o spaces>
This seems to be the (only) legit way to delete mappings2) - just recreate any mapping still needed.