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.
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