Even ignoring the threat, the sarcasm and tone of blame are unacceptable. What company would want a reputation for being mean to those choosing not to be customers (even if somehow deserved)? There was no value to Groupon in the email sent to the SF restaurant. Of course, I'm sure the sales guy felt better about himself though.
"Threat" is a bit of an exaggeration. Regardless, it's seriously unprofessional, juvenile and completely tactless. Not to mention damaging to the very company he's serving/ed.<p>I understand sales people are also human and getting hung up is bound to stir up emotions, but they still need to keep those in check while on the job. Vent at things, not people.