Mailing List CGatePro@mail.stalker.com Message #94725
From: Uwe Baemayr <cgate@baemayr.com>
Subject: Question about backup server/relaying
Date: Fri, 11 Apr 2008 19:08:52 -0500
To: CommuniGate Pro Discussions <CGatePro@mail.stalker.com>
X-Mailer: Pegasus Mail for Windows (4.41)
Howdy!

My company is moving to a new office and we'll be changing internet
providers and, therefore, IPs.  We run CommunigatePro v5.

Since we only have one in-house email server, the server will have to be
turned off, moved, and then turned back on again.  It will have a new IP
when reactivated.  

Our MIS department was planning to change the MX record just before turning
off the server/starting the move.  However, I believe, because of
propagation relays, that may cause senders who have the old MX record cached
to queue mail for the old IP, which will never be active again, and that
will cause us to bounce email.

Question: do email servers re-evaluate MX records for undeliverable mail on
each delivery attempt, or do they resolve the MX record once and then just
repeatedly try the resolved IP until the delivery timeout period expires?  
This is my impression (hope it's wrong).

We are hopeful the server won't be down for more than a few hours, but it's
always possible that there may be problems at the new location that might
extend that time.  Does it makes sense to change the MX IP 24 hours ahead of
time to minimize the chances of caching?  Or is that just not helpful?

Another thought: I do have a second Communigate server that I run for a non-
profit, and I could add an MX record pointing at that to receive and queue
mail during the outage.  However, I couldn't find any instructions on how to
configure CommunigatePro to function as a backup server in this kind of
scenario.  Here's what I was thinking -- can someone tell me if I'm on the
right track:

1) Add this record to the router table of my server:

      Relay:company.com = company.com@[xx.xx.xx.xx]

   where company.com is the company's domain name and xx.xx.xx.xx is the
   new IP.

2) Add a lower-priority MX record to company.com's DNS pointing at my
   server.

Questions:

1) Is this sufficient for CommunigatePro to receive email while
   company.com's primary email server is not responding?  Or do I have to
   create a secondary domain, company.com, on the server?

2) Will email automatically be delivered shortly after an SMTP server at
   xx.xx.xx.xx suddenly begins listening?

3) If company.com's email server subsequently rejects mail (say, for an
   invalid username), does the bounce go to the originator or to my
   external server?  

4) Is there any risk to this kind of relaying for a single domain?

Thanks for all your advice, and sorry if the questions seem kind of obvious.  
I've never paid much attention to relaying because I've always been told
that it's risky, so this is new for me.

--- Uwe


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