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Are you using redundant mail server(s) in form of an mx fallback server to collect all incoming mails if the ordinary mailserver happens to be offline?

Currently I only have one mailserver (SmarterMail v5.5) with external backup which makes it possible to restore it pretty fast on new physical server, if needed. However, in the meantime as well as if I have to restart that server, no mail is received at all - just bounced back to the sender.

Therefore I'd like to install a mx fallback server, which I've understood I can use the free version of smartermail to be. Then all incoming mail will be stored at that server instead and will try to send this mail queue to the frontend server after a period of time, and when that ordinary mailserver is back online again no mail will be lost at all for any customer.

The big question here is: How should I configure the DNS server for this - how do YOU have it configured? This also affects DNP since I'd like to set the proper DNS configuration when a new domain is added in the control panel, configured in the smartermail settings in server setup in DNP.

My frontend mailserver is using IP *.132 and in DNP server settings I currently add these dns settings for smartermail (ip address partly replaced with # signs):

mail  A ###.###.###.132  
MX 10, mail.[DOMAIN_NAME]  
TXT v=spf1 a mx -all

This way every user gets an mail.customersdomainname.tld to be used both as POP server configuration in mail clients as well as for webmail.

Now - according to SmarterMail instructions I've understood it as I'm supposed to add another MX record here with higher priority. The question is how this should look like - any experiences from this? My MX fallback will be located at ip ###.###.###.131

I'm guessing it could be something like this for another DNS post to add - however I'm not very good at DNS configuration, I have to admit:

MX 21, mailbackup.myhostingdomain.tld

Should there be any A record as well, or (if I'm right in my guess above) will all mail adressed to a mail account at mail.customersdomainname.tld automatically be forwarded to the new dns post's adress mailbackup.myhostingdomain.tld automatically, if the .132 server is offline? If so - besides this, I only have to configure the DNS server with the A record (and MX as well) for mailbackup.myhostingdomain.tld to point at the .131 server, right?

Alternatively:
Maybe I should use a dns post as follows instead:

MX  21, mailbackup.[DOMAIN_NAME]
mailbackup  A ###.###.###.131

But this maybe messes up the free version of Smartermail at the mx fallback server (.131) if host headers follows as well (!?) containing more than one single domain name (mailbackup.myhostingdomain.tld) since it would mean every customer's every domain name would follow, preceeded by the mailbackup prefix of course. I don't think this alternative is the proper way to go, but I'd like to hear what you other guys say and how you've configured your system.

I'm using MX failback. Actually higher number in MX entry equals lower priority so in my configuration mail. MX entry has highest priority.

My current DNP SmarterMail provider DNS configuration:

 mail     A    IP of main mail server    
     MX    10, mail.[DOMAIN_NAME]     
     TXT    v=spf1 a mx ptr ip4:IPs allowed ip4:another IP allowed -all     
     MX    20, mail2.[DOMAIN_NAME]     
 mail2     A    second mail backup    
 mail3     A    third mail backup (located outside server room)    
     MX    30, mail3.[DOMAIN_NAME]

Two MX backup SmarterMail instances (free) are configured just like SmarterTools tells on this page:

http://www.smartertools.com/Portal/KB/a569/how-to-configure-smartermail-as-a-backup-mx-server.aspx?KBSearchID=241021

Interesting - thank you very much for the reply.

I haven't even considered a third mailserver for external fallback as well, I'd probably look into that too. :-)  I'll try this out shortly.

on third server I have also my second DNS server so if connection to main server room fails hosts are still being resolved. I wanted to write maintenance plugin too to SimpleDNS:

http://forum.simpledns.com/default.aspx?g=posts&t=296

but their plugin class has no property which allows to check which IP is overrided during plugin operation (everything is described in mentioned URL). I hope they will add this property in future.

That was my thought also, to use an (internal) general fallback server with ns2, mail2 and www2. However, I'm using Microsoft DNS and have no experience of SimpleDNS - and as I told in my first post I'm not very good at DNS configuring :)

I had a quick look at the link you provided anyway, and have to ask if you're a developer and if so, if you've done anything with your dnp portal (the downloadable source codes)? I know a little C# and do develope a little on my own and need to do some modifications for my use of DNP, but need some guidance in simple examples of how to talk with dnp api, which I find a bit difficult to understand due to my lack of good programming skills and haven't found any good documentation from dnp team either...

I've made fewvery small changes to DNP Portal. If I want to use anything with DNP then I'm connecting directly to DNP Enterprise DB without using DNP Portal web services.