Love it! I ran into the (free) limits recently during a one-off email blast as part of a new feature rollout. The limit caused a lot of emails to fail.... but MailGun's customer service rectified it in like 5 minutes (at like 2am on a Saturday). Very impressive!

Any idea when the Heroku plans will reflect the pricing changes...?

EDIT: From the article, old plans may be granfathered (if desired)...? If so, how do we switch to new pro-rated on Heroku? Just downgrade the addon?