Errors sending through Mailgun: '421 Syntax error' - mailgun

Question

I am sending emails through a node.js app to Mailgun. I keep getting these 421 Syntax error messages come up. Sometimes the messages do end up going through, here is the history for one message:
Date/Time Summary
2015-05-07 16:14 Delivered: sender → recipient 'You have a new notification'
2015-05-07 15:14 Will retry in 3600 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error
2015-05-07 14:43 Will retry in 1800 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error
2015-05-07 14:28 Will retry in 900 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error
2015-05-07 14:18 Will retry in 600 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error
2015-05-07 14:18 Accepted: sender → recipient 'You have a new notification'
*Email addresses redacted.
Here is what the log says for the 421 error:
{
"severity": "temporary",
"tags": [],
"delivery-status": {
"retry-seconds": 600,
"message": "421 Syntax error",
"code": 421,
"description": null,
"session-seconds": 0.16810393333435059
},
"envelope": {
"transport": "smtp",
"sender": sender,
"sending-ip": "184.173.153.222",
"targets": recipient
},
"recipient-domain": domain,
"id": "TdCQ8omOSwqj_zYq18CBdQ",
"campaigns": [],
"reason": "generic",
"user-variables": {},
"flags": {
"is-routed": null,
"is-authenticated": true,
"is-system-test": false,
"is-test-mode": false
},
"log-level": "warn",
"timestamp": 1431029901.450764,
"message": {
"headers": {
"to": recipient,
"message-id": "20150507201819.16176.81911#mailgundomain",
"from": sender,
"subject": "You have a new notification"
},
"attachments": [],
"recipients": [
recipient
],
"size": 1036
},
"recipient": recipient,
"event": "failed"
}
I am new to using Mailgun and I am building the emails RAW (headers and all). A 421 is supposed to be for network errors... so 'Syntax error' doesn't make sense to me.
Some of the messages go through fine, but I do have an awful lot that are getting retried.
Any thoughts?
Thanks

Answer 1

I created a ticket with Mailgun support and they quickly helped me find the answer:
The error that you are seeing is due to the recipient's server either
1). throttling emails sent from your domain which is also known as ESP
throttling, 2) grey-listing of the IP in which the recipient server will
first verify that the sending server is not sending spam before allowing
delivery, or 3) a local server issue may have occurred such as the server
being offline or misconfigured.
The error code of "4xx" indicates that this is a soft, temporary bounce.
Whenever we attempt to deliver a message and the recipient server returns a
soft bounce, we will retry delivery for up to 8 hours in the following
intervals: 10 minutes, 10 minutes, 15 minutes, 30 minutes, 1 hour, 2 hour
and 4 hours. Unfortunately this cannot be adjusted and is hard coded in our
environment.
I checked with our admin and we had some anti-spam turned on for the mail server. We turned it off and no longer get '421 Syntax Errors'.
Thanks