Can I use a different signature in Outlook for internal versus external mail?

"Here's a 1-click solution that adds a button to your toolbar for new External messages.

Create a dummy e-mail message with your external signature to use as a template
Save As > Outlook Template .oft, e.g. External.oft > Save > Exit/discard this dummy message
Create a Macro (it's easy): Tools > Macro > give it a name, e.g. External

Don't forget to edit the path ""\Microsoft\Templates\External.oft"" to go to your template.
Optional-Give the Macro Project a short name: Right-click the top item in the tree in the left pane, i.e. Project1, and select > Chance Project name to something short like ""X"" or a non-breaking space using ASCI code Alt 0160.

Sub External()
basePath = Environ(""AppData"")
Set newItem = Application.CreateItemFromTemplate(basePath & ""\Microsoft\Templates\External.oft"")
newItem.Display
Set newItem = Nothing
End Sub

Add this to your toolbar: Right-click the toolbar area > Customize > Macros > Select your Macro > Drag & Drop to your toolbar
Enjoy!"

I'm an Exchange Administrator and you can't change the signature per se, but you can add disclaimers onto the end so you could put HTML code to an image which would then be below your normal signature, and this would only apply when emails leave your organization.