This is editable on a message-by-message basis, and you can select different signatures for different messages if you choose.
Follow these steps to choose a signature for a message.
- Go to the Messaging tab of your web settings.
- Tap the message you want to modify.
- Scroll to the bottom and select a Signature from the drop-down menu.
- Tap Save Changes.
Repeat steps 2-4 for each message so your Messaging is consistent.