Email options and configurations vary with each handset. Certainly, Blackberry is the easiest, most secure, and best "push" email device on the market. The "push" email (meaning email arrives nearly instantly, without the phone having to "poll" on a schedule) was what made Blackberry the choice of business.
Blackberry - With these devices, you use what is provided with the handset and either use Blackberry Enterprise Service (BES) or Blackberry Internet Service (BIS). If your company has an Exchange Server or uses a hosted monthly service with this capability, you will get the tightest integration of Contacts, Calendar, and Mail with your office system. More commonly, small-business will use BIS and integrate with their own email server or a hosted email service. If your business uses Alt-n's MDaemon email server, a higher level of integration can be achieved (with more on the way, as this is a Blackberry Company).
iPhone 3 or 4 - With these devices, you will more than likely configure the email client to work with your existing POP or IMAP email system or Exchange system using built-in Exchange support. Being unfamiliar with iPhone setups at this time we will have more to come later.
Android - The Android email client can be configured to talk to your POP or IMAP service or your own email server (such as MDaemon) and it will relate email to your Contact list. With simple add-on Apps, you can integrate Contacts or Calendar information from some systems using SyncML clients. We are currently evaluating these options and will have more on these at a later time. If you use Android with Exchange email, we've noted that an App called "TouchDown" is highly recommended.