How to control email flow when exchange server is down

Hi Team,
     This questions is to get more information on how the email flow works

When our Exchange Server is down how long the emails stay on the Internet and how can we control the time.

Does the TTL for DNS have anything to do with it.

If i can control it: what is the max and min time that i can hold them for before the server comes online?

Thanks in Advance
Costas GeorgiouNetwork AdministratorAsked:
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Generally speaking no, you cannot control it, since it is the other email server that will be initiating the connections.  And generally speaking, unless the other side gets a deny, no response (because your server is restarting offline, dead firewall, whatever)  will usually trigger a back off period where the email server will try again in so many minutes.
This assumes all the records in your MX section of public DNS are exhausted.  So it will try lowest and move up the chain, until the end, no response, then wait 5 minutes and try them again, still no response try again in 10.  MS Exchange will stop trying until 72 hours, though, as I remember, they all do a similar thing.  The only thing to keep in mind is the increments are additive, so the 1st is 5 minutes, the next is 10 minutes after that, not the original send activity.  So in the real world 15 minutes has passed total.

To help with things like this, for clients who feel e-mail is mission critical (and most do), I will usually suggest an email "washing" service.  They will not only remove virus and spam for me before delivering to the email server, but they will also receive and hold all the email until my server comes back up.  (different services will hold for different lengths of time, some as long as two weeks, so your results may vary)  Typically, 1-2 bucks a unique email box a month is the fee.

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With a lot of my clients I use a bSMTP server for backup, this server remains at their ISP and has a higher MX priority = so will be used less often. Mails get delivered directly to the Exchange server at the client, but in case the server or the connection is down it will be delivered to the back-up SMTP server. What happens next depends a bit on the type of server. At some clients the ISP's server will automatically try to send mails every 15 min. At other ISP's the server needs to get a message to give the 'all clear' signal and forward the mail. The regular fee I pay for that is EUR 12,-/year for the domain - not per mailbox.
Seth SimmonsSr. Systems AdministratorCommented:
When our Exchange Server is down how long the emails stay on the Internet and how can we control the time.

depends on the remote mail server
often times it might hold for a couple days; perhaps after 1 day the sending server will send a message to the recipient saying delivery delayed but it all depends on how it's configured which you can't control

Does the TTL for DNS have anything to do with it.

DNS TTL dictates how long before DNS records will refresh; not related to the actual communication process between systems
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