Okay, but this isn't a bulk email. It's a very specific situation personal to the receiver and will never be sent to anyone else. (Obviously the template will be used for multiple emails, but that's not what defines a bulk email, even though bulk emails can also be defined using a template.)
So use a different ___domain for corporate email. The only reason not to is if you are prioritizing the identifiability of your corporate email over the identifiability of your actual customer-facing operations.
If they use their main ___domain, their normal corporate email will get blocked by anti-spam filters.
So everyone uses a different, unrelated ___domain for bulk mails.