I'm running El Capitan PB6 and I've just re-set up my iCloud mail account in Mail. Previously, I've had it set up so emails sent from my account using Mail show as sent from my own email address (e.g. email@example.com) rather than firstname.lastname@example.org. From what I remember, I always set this up in Mail>Preferences>Accounts>Account Information by changing the outgoing SMTP server to the one for email@example.com, selecting "Custom" from the "Alias" dropdown and then changing the "Email Address" field to firstname.lastname@example.org However, when I select the "Alias" dropdown, the only options I get are the two email addresses associated with the iCloud account (email@example.com and firstname.lastname@example.org) and the option to Edit Aliases - the "Custom" option seems to have vanished. This means that the "Email Address" field remains greyed out so I've no way of changing it from the iCloud one. Anyone know a way around this? I think there used to be a way of doing this by editing .plist files in earlier versions of OS X, but it seems that this no longer works in El Capitan.