Changing my mx record without losing mail

We are switching from an in-house email system to Google Apps Premier.  Google Apps has a mail gateway function built-in so that I can forward all mail to my 'old' email server if a mailbox for a user isn't found on the Google servers.  This will let me pilot google apps for my workplace.  I have created my account at google apps and setup the pilot program and I'm now at the point it wants me to edit my mx record. (I just took over here less than two weeks ago so bear with me)

It says to add Google as the first MX record.  I looked up our current settings and they are as follows:

0 (our mail server)
15 (a mail list server)
20 (an external mail list manager)
30 (an external mail list manager)
40 (who knows what this is, it actually fails)

My question is... if I add the Google Apps MX record as 0 and and it's setup to forward all mail to and I set as 15.... will there be any chance of losing mail if Google is setup correctly?  

Should I just bump down the other records by 5 each?  

Why would the prior IT person put in the list manager twice?

Google wants me to add the following records:

Is it safe to do this during work hours?

How long does it take an MX record to propagate?
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When another messagings system attempts to send a message it queries DNS for the MX record and should attempt to connect to the record with the lowest preference first.  By making the change that Google requested all messages will be routed to their systems first unless they are unavailable.  You should also update the record '0' to '15'.  

You don't need to change any of the other MX records and the one that is there twice would just cause delays if the queries got to that level.

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This is what I would do.

1. I would renumber the to 5
2. I would remove the 30 and 40  (do you use groups?  Probably remove that one too)
3. I would add the google records keeping the numbering above 20


5. I would also see if you can change the TTL to say 5 minutes while you are in transition
6. I would wait 72hrs to allow the propogation of the MX records
7. After you have verified your new MX records are out there and you have everything ready to go, change the numbering of the MX records on the google DNS.
 45 (our mail server)
46 (a mail list server)
50 (an external mail list manager)


I would make this change perferably over the weekend, but if you were able to change the TTL on the domain, then you more than likely could do it during the day.  But I would monitor the old mail server to ensure no more mail is being delivered to it.  Give it a day or two and then remove the old MX records
5 (our mail server)
15 (a mail list server)
20 (an external mail list manager)

No, you will not lose your email.  This is a DNS record, and will not affect the email stored in the mail stores.  (you gotta give me credit for this one, the way that the question was asked!!!!)

As for email CONNECTIVITY, I would do ALL my MX records to point to Google simultaneously.  This will prevent mail from being delivered to two different places as much as possible.  You could turn down the TTL as biscuit3 suggested a few days before the cut over, but I would strongly recommend cutting all at once.  Me, I like to do this at like 1 AM Saturday morning.  It can take up to 48 hours for DNS records to propagate, however it rarely takes that long on small domains, as they are typically not used enough to be cached on many DNS servers.  I have seen MX records change in 5 minutes on small domains.  

Hope this helps.  
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