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Should I specify secondary MX (secondary mail server)

 
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PostPosted: Fri Aug 31, 2007 12:19 pm    Post subject: Should I specify secondary MX (secondary mail server) Reply with quote

Question: What happens to incoming mail if vqNow or vqSA is off line?
Should I retain an MX record for my mail server as a secondary server?
Asked by Peter Bromage-Smith of emrep.biz

Answer:
This raises several related questions which are addressed below:
  • 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?

Secondary MX records provide a backup route for mail in case your primary mail server is congested or offline. Sounds useful, right? Wrong! We don't recommend using multiple MX records for the following reasons:
  • The extra records show spammers how to slip around "in-stream" security services, like vqNow. [Details]
  • Users don't regularly check a backup server for mail. If a mail server is out of service briefly, they never realize that mail was delivered to another server!
  • Email protocols are very robust. Even if a mail server fails (details in #2 and 3, below), the message is not lost. It waits at the upstream server while the connection is retried periodically. This continues for 1 or 2 days. Problems are almost always corrected in this time. If the upstream server gives up, the message is returned to the sender with an explanation that a temporary failure is interferring with delivery.
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 last one below:
  • Recipient server responds but has problem with local mailbox delivery at the last moment.

    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. It may be offline or experiencing heavy congestion.

    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 or vqSA as the sending server, because it is the next upstream MTA (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.
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