AppleScript code for copying new email subject - email

Question

I'm setting up a rule in my Mail app to execute the AppleScript below when an email from a specific sender arrives. My goal is to copy the incoming email's subject line, however, this script is copying the subject of the currently selected email in the main inbox and not the arriving email's.
Is there a better way to achieve my goal?
tell application "Mail"
set _msgs to selected messages of message viewer 0
if (_msgs is not equal to missing value) then
set _msg to first item of _msgs
set the clipboard to (subject of _msg) as string
end if
end tell

Answer 1

You need to use the built-in handlers that Mail exposes to access the message associated with a rule activation; selected messages of message viewer 0 just accesses the active Mail window (as you describe). Instead, use Mail's on perform mail action handler to correctly access the message. See this question (and this answer) for more information.
Here is your code, re-written to correctly copy the subject line of incoming emails that satisfy the provided rule criteria. It's working for me on macOS Catalina 10.15.5:
using terms from application "Mail"
on perform mail action with messages _msgs for rule _rule
if (_msgs is not equal to missing value) then
set _msg to first item of _msgs
set the clipboard to (subject of _msg) as string
end if
end perform mail action with messages
end using terms from