Apple’s Mail.app still does not support multiple sender identities, such as for example “John Doe john.doe@example.com” and “Johnnie johnnie@example.net” through the graphical user interface. However, there is a hint how it is done in Apple Mail up to and including Snow Leopard.

In Mac OS X Lion, Mail has reached version 5 and Apple has introduced a new layout for Mail’s Library folder, which is called “V2”.

Here’s how you can still use multiple identities in Lion.

The “Mail” folder in the ~/Library folder now contains nothing but a single subfolder aptly called “V2”:

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% ls ~/Library/Mail/V2
EWS-john@exchange.example.com
IMAP-johnnie@mail.example.net
MailData
Mailboxes
RSS

While previously, one had to edit ~/Library/Preferences/com.apple.mail.plist, in Lion, the dictionary to edit resides in ~/Library/Mail/V2/MailData/Accounts.plist.

This file can be edited with Xcode, which is a free download from the App Store but comes in at a hefty 3.5GB. Alternatively, you can edit it in any text editor.

Be sure to quit Mail and make a backup copy of the file first.

To gain access to the ~/Library folder in Finder, hold down the ⌥ (alt) key and choose the menu command Go → Library.

In the end, the file should look similar to the one shown below (in Xcode):