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A common cause to this issue is the recipient server accepts the full message and attempts some kind of filtering on the message such as reverse dns lookups, sender id, content filtering, etc.. If for some reason if the recipient server is taking a long time to process the message then sending sever assumes that the connection has been lost and drops the connection. Unfortunately the recipient server had recieved the entire message and was just waiting to verify the validity of the email once it was verified it delivers the message.
The sending server, however, assumes the connection was lost and treats it as a 400 error and goes into a retry schedule.
If you look in the smtpo log for the message you should see a several minute delay from when the message was first sent to when the connection closed. If there isnt a delay then we may have to look into a packet capture to see if someone is closing the connection prematurely.
Keep in mind this is just theory atm as there can be countless reasons why this would happen.