If you've got the XPERIA X1 from Sony Ericsson, you may have noticed that you can't change the email signature for your default account. Well you can change it, but as soon as you reboot, it will go back to the default "Sent from my Sony Ericsson XPERIA X1" signature.

In order to remove this feature, use File Explorer to browse to: My Device>Windows>Startup. Then select the file named "SignatureReplace" and choose Menu>Edit>Cut. Then browse to another folder and paste the file elsewhere, Now you can change your signature in the Messaging application and it will stay that way after a reboot.
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