554 5.4.5 internal Delivery not attempted message expired

This is from Marketo because it did not attempt to do this. It sounds like the soft bounce was attempted within the 24 hour window, but for some reason, this came after? So it was marked as expired.

At least that's what I would think from how marketo handles soft bounces.

Thanks Josh,

I do see in the user's activity log that the email was sent June 23, whereas the Soft Bounce came June 25.

If I understand correctly Marketo was attempting to send this to the recipient, but was unable to do so within 24 hours so it became expired. It still doesn't really solve my problem in explaining why the recipients server failed to accept it.

I understand a 5.4.5 error is due to congestion. Is is safe too assume the recipients server was too busy to receive the message for 24 hours? If so, I find this difficult to believe as the recipient was a very large organization with robust email servers.

Any resolution for this issue. I see this error almost for all emails under aol.com