To remove an outdated formaly used e-mail address from the list taken into account when Apple Mail does autocompletion of e-mail addressss, open the list of previous adressees from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 …

References: Mac OS X: Using email aliases in Mail (by Apple, archived & unmaintained)

defaults write com.apple.mail AlwaysShowDuplicates -bool true

defaults write com.apple.mail _AlwaysShowDuplicates -bool true

References:
Apple Support Communities: How do I show duplicate email in Mail 6.0 (Mac OS X 10.8)
AskDifferent: How to get Mail.app to display (rather than hide) duplicate messages

(1-to-1 copy of the reference below)

1. From Mail Preferences –> Viewing, uncheck “Use Classic Layout” AND the following:
   • Show To/Cc label in the message list
   • List Preview: 1 Line
2. Close Mail Preferences
3. Your mail should now show the flag color instead of flag name
4. Go back to Mail Preferences –> Viewing, check “Use Classic Layout”
5. Close Mail Preferences

You should now see BOTH the flag icon color AND color name, but you can resize the column to hide the color name.

References:
striders at "Re: Mail flag icons are missing, shows color name instead"

OSX email