Increase the max retry time (of soft bounces at least)
In case of soft bounce it can be expected that email should go through. The current maximum retry time of 8 hours is too short if there is lots of email sent. Retry time doesn't scale well in case if most email sending is done during the office hours. Morning emails might be dropped before the less trafic nightly hours. Below is an ESP throttling response from Hotmail
ESP throttling: X → Y 'Z' Server response: 421 421 RP-001 (BAY004-MC2F19) Unfortunately, some messages from IP weren't sent. Please try again. We have limits for how many messages can be sent per hour and per day. You can also refer to http://mail.live.com/mail/troubleshooting.aspx#errors.
Travis Tarkinton commented
On Friday we had the fiber line cut to our location that houses our Exchange server. It typically takes 12 hours to fix a line cut like that (this has happened to us a couple of times before). Because our fiber line was cut, we had no internet to our Exchange server, and several emails sent through Mailgun permanently failed as the outage was longer than the max retry time of 12 hours. We tried to resend these emails this morning through the Mailgun interface, but ran into the 3 day retention expiration. Our suggestion is to bump the time limit for max retries a little bit and to increase the retention period an extra day in case of a weekend (or long weekend) where failures aren't caught in time.