Reverted to using the previous configuration for now.

Seems like a good option is adding a roundup-tracker.org configuration to our mailgun account, which would just require a couple DNS 
entries to be added on the zone:

smtp._domainkey.roundup-tracker.org IN TXT k=rsa; 
p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAyKRxsc7RPZD6wH83707zeLF21mtQWjxKc9Bb8DhqWBQ4U3jQEf/xiMK3pLl
gcMhNspeWywTgwTv80xczsFyqiU6aULQZZSemY3abRsPbic7XwTXku3U3pUp8l6FGgC3i+NCIHNpE+p53maziKfAB7kWb0VgjYbbpsm+1
sWcSEilYWgAccI9HyZmsWirl5ipA3mPDBmheLdv8v2xshepWN4xLplPd4aoAMpDjYvmtd/UEnlnj73zJUyfJNxyCt9P+Xz7eKBgJ7Z1KHRQF
3tON3HgYHbhr5Ddq/bT8GpjMPVMu/7m7WmKxgLVJB6FntI06wRuXsgBSsMxlvOLZ1r0vHwIDAQAB
roundup-tracker.org IN TXTv=spf1 include:mailgun.org ~all
email.roundup-tracker.org IN CNAME mailgun.org

Then we could configure all emails from the instance to send on the roundup-tracker.org domain only (remove all references to 
python.org), leaving MX records as is to allow the email gateway to continue working.