Issue Summary

Email delivery from cloud instance is delayed and does not get delivered because Sparkpost is not trying to connect to the secondary MX if the primary MX of the mail server is unresponsive. We can see the following error reported in the logs at our end:

451 4.4.1 [internal] No valid hosts (unable to make any connections)

Steps to Reproduce

1. Setup primary and secondary MX record for your mail server
2. Make sure that you cannot connect to the primary MX record

Expected Results

When primary MX record is not responsive, Sparkpost should connect to the secondary MX record of the mail server to try to deliver the emails

Actual Results

Sparkpost continues to try to connect to the primary MX to deliver the emails and does not try to connect to the secondary MX

451 4.4.1 [internal] No valid hosts (unable to make any connections)
OR
554 5.4.7 [internal] message timeout (exceeded max time, last transfail: 451 4.4.1 [internal] No valid hosts (unable to make any connections))

Workaround

Update the working secondary MX record as primary MX after which the emails will start getting delivered

Notes
We are currently working with Sparkpost to resolve this issue