Keeping Emails If Server Offline

I'm thinking of hosting a mail server behind a Dynamic DNS service.
1) Is that possible?
2) If there any failsafe in smtp that allows for incoming emails to be kept if the mail server is down?
Silas2Asked:
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BenefordCommented:
You're likely to lose emails if the connection goes down and is then reconnected with a different IP address (which, on a dynamic IP circuit is very likely).

There is a TTL (time to live) on the dns lookup, and a mail server trying to connect to you will assume the IP Address remains valid for that TTL period (even 10 minutes is too long for what you need).
If you do drop the connection the mail server may succeed in sending email addressed to you to another server (which just happens to have connected with your old ip address), and that server just might do something with it.
If you want to be secure and not lose emails, you need a fixed ip address.

If your system is not highly available (you need to define how high it needs to be - in-office or at-home broadband/fibre pretty reliable, but it's not 100%) you probably want a backup Mail Exchanger that will store email while your primary server is down and forward them when you come back up.

Some ISPs provide this service along with a hosted server (I have two cpanel hosted servers that do the job).
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Silas2Author Commented:
Thanks for that. Regarding "...you probably want a backup Mail Exchanger that will store email while your primary server is down and forward them when you come back up...." how would you enable that to take over receipt automatically?
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BenefordCommented:
Define two MX records with different priorities.
The lowest priority is for your primary server, and a higher priority for the backup server.
A mail server will normally locate your lowest priority server and use that; if it's not available, the higher priority server will be used.
If correctly configured, the backup mail exchanger will also check for the availability of the higher-priority server and forward mail to then when it becomes available.
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Silas2Author Commented:
I've just configured an email server (with postfix) would the back mail servers need to be setup with the same ? i dunno what, settings? to take the mail due for the first server?
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