How to place your signature above or below quoted text
By default, Postbox will place your signature below the quoted text, but you can make it place your signature above the quote. There is a separate setting for each account and identity, where you can configure where your signature is placed.
Go to Preferences | Options > Accounts > [your account] > Composition to set the default signature location.
If you have multiple Identities, go to Preferences | Options > Accounts > [your account] > Identity > Manage Identities button > [your identitiy] > Composition.