What happens to mail if vqNow is off line or unresponsive?

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Question: What happens to mail if vqNow is offline or unresponsive?
Asked by Peter Bromage-Smith of emrep.biz

Answer: The answer touches on 3 interrelated issues:

• Should I specify a secondary server?
  
• What happens if vqNow fails or is off line?
  
• What happens if my own mail server is off line?

1. Should I specify a secondary server?

If the lowest numbered MX server is not available, then the sending service moves on to the next higher numbered server. So in your example, if www.vqnow were not responding to a connection (or if there is a failure of "peering" on the transcontinental backbone, then it would move onto rescue.mx.dnspark.net.

However, we don't recommend specifying secondary servers because spammers can use this information to get around your security pipeline! [detalils].

2. What happens if vqNow fails or is off line?

This leaves your original question: What happens if the listed server(s) do not respond to an incoming connection. The SMTP protocol is quite robust in such cases. In fact, the worst case event of mail being totally lost is virtually unheard of. It only happens in the case of human error. Let's look at various scenarios. Your question pertains to scenario #3.

• Recipient server responds but has problem with local delivery
  In this case, the sender is informed of the problem but that the problem is temporary (For example, the mailbox is full, or the message is too big, etc). In any event, the sender is told of the status and also whether they need to resend it.
  
  
• Recipient domain is invalid - or has no DNS record
  In this case, the sender is informed of the problem and the entire message is returned. This usually indicates that the sender typed the wrong address, or it could be a problem with the DNS lookup records of the sending service. In either case, the rejection messages is sufficiently informative to help the sender correct the problem quickly.
  
  
• Recipient domain is valid, but the recipient mail server is unresponsive. (off line or heavily congested)
  In this case, the sending service knows that the domain is valid (their DNS server has confirmed the address), but no one is responding to the "doorbell". In this case, they cannot determine the nature of the problem.
  
  If there is no secondary server listed on the MX record, the sending server (or the next one in the delivery chain) will "back off" and then try again. The period of back off is randomized but gradually lengthen's from a few minutes to a few hours. During this period, the sender is typically notified of the problem after 1 day. Some services notify their users after 4 hours, but if they do, they typically inform the sender that they needn't RE-send the message because delivery will be retried automatically.
  
  Most sending servers give up after 24 or 36 hours. But some will keep trying for 5~7 days.

3. What happens if my own mail server is off line?

In this case, think of vqNow as the sending server, because it the next upstream message-transfer agent. Our policy is to try for 36 hours and then return the message to the sender. Since we do this after just 36 hours, we do not send out earlier warnings about the problem.