Site migration and email servers

I just landed this project with a company to help them migrate their websites from one hosting company to another. Migrating the site should be easy enough, however, what happens to email when the DNS settings are changed?

They have about 100 email accounts with their current network provider, who they are keeping for LAN/email support services, and they want to reduce their web hosting costs by going with a different company (hosting provider).

They are all using MS Outlook for email.

Now, if I change the DNS settings for the web site, does that effect the routing for email as well? If so, how do I migrate the existing archives in outlook? I would assume that if the archives are set-up on local hard drives, it's just a matter of re-establishing the links, but if they archive their emails on a POP server, how do I migrate that data? Can I just change the DNS settings for the website and keep the existing settings for email so I don't have to worry about migrating the data? I seem to recall that this is possible, but I'm not sure. Any ideas?

Are the email accounts also with the hosting company that you are changing? If you are changing email servers as well then you are in for a lot of work as those accounts would then have to be created again at the new host.with the new email server.

"They have about 100 email accounts with their current network provider, "

so that provider handles all of the email? - if so its just a case of adding MX records for the primary and backup email servers, although if the current DNS already points to the right place you need only change the IP for the web related stuff... basically its actually easier for you for them to keep the existing mail setup, you will have to do no more than point mail in the right direction, the alternative would mean setting up al the pop accounts at the new host (and redirects etc) and then changing everyones outlook setup, so you are getting off easy :)

Yes, they want to keep their existing email server/provider. So when I get their new web hosing set-up, I just need to tell the hosting company to modify only the portion of the MX record relating to the website and tell them to leave the email stuff alone? Will that do it?

I guess I should read more closely. :)

Just thought I'd give everyone an update on this. I contacted the new hosting co, and asked if this was possible.

They said that as long as they have an IP address to point to, and that the computer/server at that IP is already configured for email (using MS exchange or similar app) then it only involves a simple modification to the MX record for the domain.

Man that makes my job really easy! Whew, dodged a bullet there. Thanks for the help everyone