Use an email plugin and control the email sending directly from outlook. AFAIK what you're trying to capture (error code 1 - user cancelled action) is only available inside of filemaker, and doesn't link back once the blind email command is sent to your email program.
Using a plugin with a send button inside of filemaker gives you complete control over the email process. There are plenty of plugins that support SMTP sending.
Once you send out the mail data to an external Mail program, it is beyond the reach to catch an error.
If you mail directly from a server, then you can catch errors, except error 1 because the user cannot cancel.
So you need to add a script step to request confirmation from the user, then send it via an SMTP server.