The alternative would be to not include "RE:" in the subject, which makes it looks like a reply to an earlier thread. The subject is subject enough already. Don't manipulate it with a "RE:" to imply something more.
That said, I've been working with Twilio on and off for 2 years and have never received an email like this from them.
Looks like this is a bit of email usability fail on our part. When you first sign up for Twilio, you receive an email with the subject line "Welcome to Twilio... Learn to Build Your First Phone App in Less than 5 Minutes" and then 7 days later we send you an email introducing our CEO that says "RE: Welcome to Twilio..." in the subject line. The goal was that these would get threaded together, but they don't since the from is "Twilio" in the first and we use real names in the other emails (the one from Jeff says his name, the one from me says my name, etc.)
Sorry about that, it's an easy fix to remove the RE:
That said, I've been working with Twilio on and off for 2 years and have never received an email like this from them.