To allow an Exchange user to send from an external e-mail address, perform the following steps:

1. Create a Mail Contact with the correct external e-mail address in AD and in Exchange.
2. Open Exchange Management Shell and set up the permissions:

   Get-MailContact MyUser@MyExternalDomain.com | Add-ADPermission -User MyUser@MyLocalDomain -AccessRights ExtendedRight -ExtendedRights "Send As"

   Replace the first e-mail address with the external one, the second part contains the user you wish to assign the permission to.

By default, Thunderbird shows the namespaces of IMAP folders as a seperate folder containing the subfolders belonging to that namespace. An example of this is when you use Google Mail and have IMAP access configured.

An add-on exists for Thunderbird which gets rid of the namespace folder and sorts all subfolders in the root tree:

https://addons.mozilla.org/en-US/thunderbird/addon/flat-folder-tree/

This is particularly useful for Google Mail where the sent mail, trash and other system folders are stored in a namespace called [GMail]. This way, these folders are stored directly in the root of the tree rather than in a subfolder named after the namespace.