Email Server Set up

Question

Ok, I'm not new to Macs but I am new to setting up the emai service.  I just took the Server Essentials course and learned how to set up an email server using OS X but I am unable to take what I've learned and use it in our environment.  A little background: 
We're running OS X Server 10.5.8 on a 2x2.3 ghz Quad-Core Intel Xeon Xserve with 6gb of RAM. 
Our mail is POP, hosted by a hosting provider. 
We do have a VPN connection.
What I am trying to do (or to find out if I CAN do this) is set up a local mail server to capture all email so we don't have to continuously backup the individual workstations.  It would also be very helpful to have a central Address Book.  In addition, most of our users are laptop users (all Mac) and may need to connect remotely. 
So my questions are these:
How do you set up a mail server for an existing hosted POP account?
If you set up a mail server, how do people get their mail on remote devices, like phones, iPads...?
Would a centralized mail server allow for searching of individual emails?
Any help I can get here would be greatly appreciated.  I'm an IT manager but I'm expected to know everything about everything (including electrical work).  Every other place or person Ive gone to has been thoroughly specialized and not able to help me with my specific needs.

Answer 1

You would have to transition all your users to the new mail server hosted in-house. You really can't have two separate email systems (other than in a redundancy config) running together. There may be some possible kludge to make that work, but it's probably more work and risky than just hosting all email internally.
DNS will have to point to the proper machine and MX record for mail to flow in and out. My recommendation here is to do this over weekends when no one is usually working and to get DNS working first before moving on to running mail services. This all does pre-suppose that you have a registered domain name (example.com) for this to work, yes?
Based on your requirements, as Camelot pointed out, I would go with an IMAP-only setup. Emails from the POP server/accounts can be transferred to their IMAP account. When I did exactly what you are looking at, I had all users create a NEW distinctly named IMAP email account in their mail client, because it is very probable that your old server will still be active and accessible until you terminate your email services with them. Users can then move their emial folders and messages to your internal server right in the mail client. After they have moved their messages, they can disable the old POP account and run solely on your email server.
Mail storage files can get sizable regardless of the system environment. Email attachments are the real killer in IMAP configs: big attachments, or lots of smaller ones, balloon the email store directory usage. Consider implementing quotas and attachment size limits if is seems appropriate for your environment. (Just yesterday I had to ask one of my users to prune out his emails - his mail store was in excess of 9GB - and threatened quotas if he didn't get a little more proactive on weeding his email folders regularly.)
Would a centralized mail server allow for searching of individual emails?
Only to the same extent that users can search within their own emails. Searching accross accounts - like Tom searching in Jim's emails - isn't allowed at the user level. At admin level, on the server, you could do some grep searches in command line but even that's a little dodgy given the sacrosanct nature that cyrus invokes permissions-wise on contents of the imap/user/ directories.
so we don't have to continuously backup the individual workstations
That works, only to the extent that you are backing up your server. (And Time Machine backups of/on your server and NOT a bulletroof backup solution.)  Even then, not having clients backed up is not a wise strategy either. Time Machine is fine at the user/client level, but have a robust backup strategy for your server in place.
HTH,
-Doug