At our (small) office we're deciding whether to have our MX record point to our local Exchange server. I was a bit worried about downtime, so thought that I ought to set up a backup MX server. Then I did a bit of research, and found a couple of posts that suggested it wasn't necessarily worth the effort, most notably because the sending MTA will queue the mails for a couple of days before giving up, which should be more than enough time to get your affairs in order.

I was all happy to go with that, but our local IT provider who we're buying our new server off insists that servers don't always queue the mails: sometimes they just immediately bounce. Can anyone comment on this position?

Also, I'm trying to find what the standard says on the matter (though I'm aware that real life and standards aren't always in alignment). The Wikipedia article on backup MX records says

The SMTP protocol establishes a store-and-forward network, and if a domain's mail servers are all offline, sending servers are required to queue messages destined for that domain to retry later.

Looking at RFC 821 I can't find that part, but I'm only scanning. Could anyone point out the relevant part if it exists? I'd also be interested in any part dealing with how long the mails ought to be kept for before discarding.

3 Answers 3


The IT provider is right, some do bounce immediately. However, in my experience it is only dumb-hosts that bounce immediately, smart-hosts will queue. There are some de facto standards out there for how long an MTA will queue mail before creating a DSN, with 4 hours being the most common. As with all such standard, there is a lot of variability.

The kinds of mails that tend to get sent by way of dumb hosts are mail coming directly from web-applications who are counting on your server being smart enough to deal with exceptions. These can be your own applications to mass-mailers out there on the web. These messages are generally of a lower class of email, but not always.

If you do decide to go with a backup MTA you will NEED to put anti-spam on it. Spammers have known for over a decade now that 'backup' MTAs are a great place to target since they generally aren't as well protected as the primary.

  • +1 By far, most MTAs queue, and the time they'll hold the e-mail is around 4 hours (commonly more). There are others out there, but your mail server shouldn't be going down frequent enough for it to be noticed/problematic (unless you've got some crazy SLA requirements; or a really buggy server).
    – Chris S
    Nov 19, 2010 at 15:27
  • Thanks for that, I guess I don't want to risk having just our local server as the MX server. Not sure whether to get a backup MX or just have Exchange pull from our main server. And thanks about the spam warning - that's exactly what the articles I've read have warned about, and they say it can be quite a bother for various reasons.
    – Andy
    Nov 19, 2010 at 15:28
  • Actually, I think it makes sense to use a service like DynDNS.com's MailHop relay. This just acts an intermediary, scans for spam/malware, forwards to your mail server if available, and queues if not.
    – Andy
    Nov 19, 2010 at 16:12

I don't think it is required, but it is good practice.

Pretty standard values for queueing and retries are "first retry after 30 minutes, then every 60 minutes until 12 hours have passed; after that, every six hours until 72 hours have passed, after that, a few more retries at 12-24 hour intervals. If mail cannot be delivered in 7 days, drop the email and (optionally, these days, as there's far too many faked originators) return a 'Sorry, could not deliver'".

This is, admittedly, mostly from experience and somewhat dated, as I haven't run a mail server for work purposes for about 4 years now.


Mail servers SHOULD queue, but not all Mail servers behave nicely. But since there are so many other things that can go wrong with email, most very small businesses I know don't bother with a backup as it does not seem cost effective.

If you want to do so, you might want to check if your ISP will act as a store and relay backup mail server, which means that they will accept the email on your behalf and send it on to your servers when it becomes available. (Doesn't help if both your server and the backup servers goes off-line).

From our own experience, our ISP offers this service, but it is an extra cost which we decided not to take up. (It hasn't caused a problem in over ten years and since our customer base is very small and non-technical, I'm fairly certain they would lift the phone if they had problems sending email to us).

  • Thanks, yeah we have reseller accounts with Fasthosts and HostGator. I know they definitely don't do backup MX servers (unless of course you would like to purchase a dedicated server!). I've opened tickets to see if they do anything like DynDNS.com's MailHop Relay I describe above. DynDNS only charge USD 50 per year, so that's very affordable, but free would be better;)
    – Andy
    Nov 19, 2010 at 16:33

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