Ok this is a question that needs urgently answering for the company I am with..

We are transfering a domain name and their hosting for a company and they have many emails so they need to know what is going to happen..

So when I transfer the domain to this companies registrar account, the name servers will be the same.. so everythings still ok.. when I change the name servers to our server.. it will take 24-48 hours to propogate.. so what downtime will be involved and what emails will be lost?

There should be no downtime. It takes 24-48 hours to change the nameservers and get them updated, but the minute this actually happens, then everything will be transfered over. There will be no "downtime" as such - one minute all emails and traffic will be going to old nameservers, the next they will be going to the new nameservers.

It takes 24-48 hours to update, but the actual transfer is immediate.

Thank you. Exactly what I thought. Done transfers of domains and hosting plenty of times past few years, however this client is rather respected by this company so needed to confirm.

When you change name servers you should also set the TTL (Time to live) of your domains to a low value, 15/30 mins, set this a few days before the change to let it propagate and then when you plan to change name servers then it should update very quickly, On the new name servers it is probably a good idea to also set the same TTL in case you need to make some quick adjustments. Then after everything is in place set them back to 12/24hrs whatever your preference is.

there could be some lost emails. For example if someone sends an email to your domain after you make the namserver change (and their ISP is using cached DNS which most all do) then the email could be sent to the old IP. Normally ISP's clear out their DNS cache every 24 hours. To get around this, setup 2 profiles for your email within Outlook. One to check the old server's mailserver and one to check the new mailserver for mail.

You can remove the old Outlook profile in a few days just to be sure.

Typically you're not going to lose any emails but if it is mission critical you should setup the 2 profile (1 old, 1 new)