Stop using blacklisted servers to send mail for fixed IP accounts
Fixed IP accounts should not use random server IPs to deliver mail when current server load is high. This results in emails not being delivered because the sending server IP is often blacklisted. Fixed IP accounts help improve sender reputation, and this is negated when mail is sent by a server that is blacklisted.
Issue is stil remained
We pay for a fixed IP, but Mailgun does not honor that. Either the fixed IPs should be distributed over more servers to avoid high loads, delay delivery until load is lower, or make sure the alternate servers are not blacklisted.