Email can be complicated on handheld devices and there are often many options. Things to look at include:
Here are some notes.
The Pocket Outlook ("Messaging") has 3 ways of getting email; POP3, IMAP4 and Exchange. You may setup multiple accounts. POP3 and IMAP4 are defined on the phone, but an Exchange account (called "Outlook E-Mail" on the Pocket PC is setup on your desktop via ActiveSync. I've used IMAP and Exchange a lot, but not POP3.
This works fairly well. You don't get contacts and calendar information this way because those things are not part of the IMAP protocol, and only messages from your Inbox are forwarded. However, you do get your full folder list, and you can move messages to different folders, and those changes are reflected on the mail server. IMAP may be set to connect every x minutes, the minimum being 1 minute.
This works in conjunction with ActiveSync; you may select Calendar, Contacts and Email to be synchronized with your Exchange over a wireless connection, or any of these may be sync'ed with your PC when the device is docked. You may also select which folders to sync with the device which is useful if you have server-side filters moving mail to different folders but still want to see those messages on your Pocket PC.
Oddly, the schedule for sync with the server or over wireless is a minimum of 5 minutes. In other words, this is not a full Exchange client that gets mail messages pushed to it as they arrive. That may make sense for a phone, but not for a device with an 802.11 connection. On the other hand, it appears to use port 80 (or 443 if your Exchange server has SSL setup), so you don't need VPN to get past your company's firewall.
Email is similar on the Microsoft Smartphone with a few important differences. IMAP4 and POP3 accounts can be defined, and there is a SMS "account" for SMS messages. Synchronizing with an Exchange server is configured the same way as with a Pocket PC, on your desktop with ActiveSync. But calendar, contact, and mail changes can be set to synchronize "immediately" or "as items arrive", if you have Exchange 2003 or the Mobile Messaging service on Exchange 2000.
How this works is amusing. When the wireless schedule is set to sync "When new items arrive", new email (or calendar or contact changes) cause the Exchange server to send your phone an SMS message (you have to enter your phone's email address the first time you set this). The SMS message is intercepted (you never see it or hear a beep), and the phone will make a data connection to sync with the Exchange server. If you are paying per SMS message, this might rack up some charges.
In my case I keep Outlook open on my desktop computer with the iHateSpam plug-in running all the time. This moves spam to the Junk Email folder as it arrives. But since we don't have the version that runs on the Exchange server, the server sees new mail, and sends the SMS message to my phone, which then makes a data connection, but doesn't find new mail. That's great, I don't want the spam on the phone, but it makes for a lot of unnecessary data connections.
The "Wireless Schedule", configured via ActiveSync on your desktop, can define how often to sync (manually, as new items arrive, 5 minutes, 10 minutes, etc.) for peak times, off-peak times, and when roaming. The "Peak Times" default to M-F, 8am-6pm, but can be changed.
One very annoying limitation on the Smartphone is that your email folder list is fully expanded when you display all folders. On the Pocket PC's, subfolders can be opened and closed by tapping the "+" or "-", as you would expect for any tree-like display. The Smartphone does allow individual folders to be selected to be shown or not, so if you are synchronizing a few subfolders these can be selected to be shown when you "Hide Unused Folders", but moving messages to your various folders remains painful.
I used the RIM Desktop Redirector to forward Exchange email to a Blackberry 6710 for more than a year. This is described in detail here. An important difference between this and IMAP or ActiveSync is that this setup forwards email to the device; it does not sync email. So, once a message is sent to the device, if you delete or move it via, say, Outlook on your desktop, the message will stay on the device until it is docked.
This lower level of functionality is useable, but the main problem I have is that it forwards messages before spam is moved, so it gets all the spam. Another big limitation is that moved or deleted messages, and replies, are not reflected on the Exchange server until you dock the Blackberry and sync it.
Another limitation is that you cannot select folders, in addition to the Inbox, to forward messages from.A minor annoyance is that the Blackberry, with the Desktop Redirector, uses email messages to forward email messages to the device and to send replies to via your Exchange account. If you have Outlook open on your desktop you'll see these "relay messages" flash in and out of your inbox.
I assume the RIM Enterprise Server, something you buy and add to Exchange, works better, but it is expensive and they don't offer a free trial, so we don't know how well it works.
This is a desktop application plus a phone application offered by Verizon for various kinds of phones; I used this for a while before we had Exchange 2003 setup. It seemed to crash the phone a lot, and didn't work very well. It is similar to the Blackberry Desktop Redirector, where the PC application sees new mail messages, sends them to a web service which attempts to contact the phone to tell it to sync with the web server. Mail, calendar, contacts and tasks can be sync'ed. It worked some times but often did not. If the web service could not reach your phone (e.g., you had no signal), it wouldn't try again until you got another email. There was an option to sync manually from the phone. This often did not work either.
More details are here.
Various devices that have web browsers might do email with a web-based email service, such as PSU's Webmail, Yahoo, Outlook Web Access, etc. These work if the device has a large enough screen.
Outlook Mobile Access is designed for small browser screens; we've not be able to make that work yet.