Randomize timing of bounce retries to reduce multiple bounces.
If I attempt to send 100 emails to users at a single host and am rate-limited by the recipient's e-mail host, the mailgun system attempts to resend those e-mails after 10 minutes. The result is that the same 100 emails all retry at the exact same time and nearly all of them bounce again. Subsequent retries with longer delays will continue to bounce as they all attempt at once.
If retries were randomized between 5-15 minutes, deliverability in this scenario would be substantially improved with minor development effort.