Here is a snippet of code showing how to create an email message with an attachment using the Mail application.
use AppleScript version "2.4" -- Yosemite (10.10) or later
use scripting additions
property emailSubject : "Your Email Subject"
property emailSender : "Your Name <your_address@domain.com>"
property recipientName : "Someone"
property recipientEmail : "someone@domain.com"
property messageBody : "Hello,
This is a test message generated by AppleScript.
"
set attachmentFile to choose file
tell application "Mail"
set default message format to plain format
tell (make new outgoing message with properties {subject:emailSubject, content:messageBody, sender:emailSender})
make new to recipient at end of to recipients with properties {name:recipientName, address:recipientEmail}
make new attachment with properties {file name:attachmentFile} at after the last character
-- Uncomment to actually send the message
--send
end tell
end tell