SMTP hard bounce

Posted on 2006-11-07
Last Modified: 2009-12-16
I am running SBS/Exchange 2003, and am currently connected to a UPS with auto shutdown in the event of power failure.  My question is:  should the power fail for, say 2 days, what is the typical bounce policy?

For example, someone sends me an email.  The power is out.  Their SMTP retries sending the email multiple times until it succeeds or fails.

I've read it is typical that the other server will try several times for about four hours.  After 5 days it quits trying altogether.  Is this correct?  What are your experiences with these types of bounces?  Should I be paranoid about a short power outage (say 8 hours), or do the other SMTP servers usually do a good job of re-sending hard bounced emails.  A generator is not in the budget.  Thanks.
Question by:bleujaegel
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Expert Comment

ID: 17896572
Bounce policies will vary depending on the whims of individual mail administrators. However a properly configured mail server will keep retrying delivery for several days before bouncing the mail.

If you think you might have outages longer than 24 hours or so, it's probably a good idea to find someone to run an offsite secondary MX for you which can have a longer retry period.
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Expert Comment

ID: 17896608
If you have no power for more than 24 hours then I operate the policy that you have bigger things to worry about.
I can have a replacement email server (not always Exchange, but email) available in less than 2 hours.

Most email servers will try to send email for 48 hours and then hard fail. Very unusual to continue for five days. If an email cannot be delivered in 48 hours then most servers will consider that the server has gone and give up.


Author Comment

ID: 17896611
How does an offsite secondary MX work?  Are they esentially a stand-by mail host?  If so, how do you synch up Exchange if email gets split between 2 servers?
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Accepted Solution

dynamitedotorg earned 250 total points
ID: 17896624
An offsite MX will accept mail on your behalf and attempt to forward it on to your primary mail server. If you think you are likely to have lengthy outages, you can configure it to retry mail delivery for as long as you want thereby minimising the chances of losing mail.
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Assisted Solution

Sembee earned 250 total points
ID: 17897355
I am not a great lover of backup MX services.

The main reason for this is that the most effective way of dealing with spam is to block it at the point of delivery. If you are using another server for email then it will already have accepted the spam message and you are obliged to accept all email from that service. You then have to drop the spam internally, using bandwidth to get it delivered.

Furthermore, most backup MX services will want to be a lower priority in the MX records to reduce the amount of email that they send, but spammers will target those servers for their messages because they are often not as well protected as the primary server and there is a higher chance of their message getting delivered.

If you are in an area where a 48 hour power outage is likely then you should take other precautions, such as moving the server to a data centre.
Another option would be to use a dynamic DNS account for the second MX record that is pointing at the original server all the time. In the event of a failure you simply change the MX record to another host and email will continue to flow.

Don't make changes to your environment that limit the day to day operation of your network to cover yourself for an event that MIGHT happen maybe once a year. That is just plain daft and will cause more pain than the event you are looking to protect yourself against.


Author Comment

ID: 17900507
>>An offsite MX will accept mail on your behalf and attempt to forward it on to your primary mail server.

If I could configure this, it would be an acceptable solution.  I like the idea of having another server receive the email in the event of a failure, and having it forward it to you at a later time.  And of course being able to configure the retry interval is a deal maker.

I also like the dynamic DNS idea.  I will keep both in mind.  Thanks for the help to the both of you.

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