To remove an outdated formaly used e-mail address from the list taken into account when Apple Mail does autocompletion of e-mail addressss, open the list of previous adressees from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)
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"
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.
References:
GPG Tools Support: How to add an e-mail address to an existing public key using key mapping
Move files matching
Library/Containers/com.apple.mail//Data/Library/Logs/Mail/*.txt
to some archive.
Select Menu Windows → “Verbindung prüfen” (sorry, only german installation at hand, something like “check/verify connection”)
In the new Window (de)select “Protokoll für Verbindungsaktivität” (“Log of connection activity”)