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Summary:
Using an AppleScript to create a message and then set the signature on that message fails and gives an error message of -10000

Steps to Reproduce:
Open Mail.app, create a signature (with a name of “Test Signature” for example).

Open Script Editor and create a new document with the following text:

tell application "Mail"
	set messageInfo to {subject:"Test Message Subject", visible:true, content:"Test Message Content"}
	set newMess to make new outgoing message with properties messageInfo
	tell newMess to set message signature to signature “Test Signature"
end tell

Expected Results:
A new message is created and opened in Mail with the Subject “Test Message Subject” and content “Test Message Content” and the signature set to “Test Signature”

Actual Results:
A new message is created and opened in Mail with the Subject “Test Message Subject” and content “Test Message Content”. However, the signature is not set at all and an error is returned in Script Editor (error "Mail got an error: AppleEvent handler failed." number -10000)

Version:
Version 10.3 (3273)

Notes:
This has been tested without any plugins installed and is current in both 10.12.5 and 10.13.