mx record for linux mail server

Dear Experts:

As of now we have outsourced the mail services to the web hosting company and we have a domain control panel login details where the mx pointing is done for the outsourced web hosting company , now I want configure the mail server in the office ( in the LAN) using the zimbra collaboration suite (open source),  I have time of 30 days to completely setup the zimbra mail server up and working but I have few doubts

1.      while installing the zimbra collaboration suite (zcs) it should have a valid MX record, if I Change the MX record to point it to the local IP of the server which is in the office then at one stretch I have completely depend on this new server but for now I only want to install zcs in the office server for the same domain which is outsourced to the webhosting company but there should not be any down time in the mail services. Later after 30 days will completely depend on this new server of the office.
2.      I want complete the installation and configuration part for the same domain which is outsourced on the 30th day this new server should start working.

Please suggest me how to achieve the above also request you to give a suggestion for such implementation.

Thanks in advance.
Who is Participating?
Chris DentPowerShell DeveloperCommented:
1. If you were to dis-allow inbound SMTP to your new system, then add an MX record for Zimbra, e.g.  MX 10  MX 20

This way, your MX is ready and will be fully propagated, but mail will not be delivered to Zimbra (yet) because nothing can get to it.

Risks? None really, standard behaviour is to try the lowest priority MX first, if that fails fall down to the next responding MTA, and if nothing responds, queue for retry.

2. Open the firewall at this point.

Depending on how your system is set up, and how flexible things can be, leave it like that for a couple of days for MX propagation then remove the hosted server from the MX.

You can reduce propagation delays by reducing the TTL of your MX record entries, so that "couple of days" can be couple of hours if you prefer without incurring any risk.

Chris DentPowerShell DeveloperCommented:
Just for clarity:

> dis-allow inbound SMTP

I mean block TCP/25 inbound (or rather, don't open it yet).

Only open it when your system is ready and you're happy to switch live-services.

D_wathiAuthor Commented:
Sir, Thanks for the reply, one more final help please that is in the domain control panel will add the mx slightly higher value  like the below as current mail hosted has the value of MX 10  MX 20
Now iam going to configure the mail server behind the firewall that is we are on the NAT network hence required to configure the split-dns setup as per the documentation of the zimbra.

1. My doubt is what value should be set in the split dns as in the domain control panel the new zimbra mail server value is set to 20, should i give the same 20 value in the split dns configuration of the zimbra mail server ( new ubuntu mail server in the office). Please suggest. Thanks in advance.

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