Hey Jack,
This is a known issue, but the fix has not yet been determined.
In the meantime you can script it directly.
-Chris
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# Auth: Christopher Stone <scriptmeister@thestoneforge.com>
# dCre: 2013/05/15 06:11
# dMod: 2015/10/08 14:33
# Appl: Apple Mail
# Task: Make a new outgoing message.
# Libs: None
# Osax: None
# Tags: @Applescript, @Mail, @Make, @New, @Outgoing, @Message
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set toName to "John Smith"
set toAddress to "address@null.com"
set _sender to "Christopher Stone <null@null.com>"
set _subject to "Test Message"
set _body to "Now is the time for all good men to come to the aid of their country."
set _sig to "Take_Care"
tell application "Mail"
set _sig to signature _sig
set _msg to make new outgoing message with properties {subject:_subject, content:_body & return & return}
tell _msg
set visible to true
make new to recipient at end of to recipients with properties {name:toName, address:toAddress}
set sender to _sender
set message signature to _sig
end tell
activate
end tell
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