Stuart>> If you only have one Mac, and no need to sync data between Mac, I think GMail/Google will meet your needs for free. I will give you push email on your Mac and iPhone and sync calendar and address book data between the two over the air.
I put the following together to help a friend recently and he likes the setup.
==============
http://mail.google.com
Go here and sign up for a new account and write down your user name and PW. Then on the Gmail web page go to settings and the "Forwarding and POP/IMAP" tab and enable IMAP at the bottom. Click save changes at bottom.
Go back to Gmail settings to the Labs tab. Scroll down to the setting for "Advanced IMAP controls" and enable that setting. Save at the bottom.
Go back to setting and labels tab. It will show a list of "system labels"... this is GMails version of folders in your email client. to the right of the label for Chats uncheck the "show in IMAP" setting... also uncheck show in IMAP for the "All Mail" label. That sets up the GMail side off things.
Also click on Calendar in Gmail at top left to activate GMail calendar functions.
Now to set up Apple Mail go to this link and follow the instructions. If you have important saved emails you can forwad them to your Gmail account to save them. Be sure to click the link in step ten of the instructions and configure the Apple Mail prefs as it says.
http://mail.google.com/support/bin/answer.py?answer=81379
Once your Gmail IMAP account is added to Mail, youll notice your [Gmail account] in the left sidebar.
Highlight [Gmail] Sent Mail in the sidebar and select Mailbox (menu bar) > Use This Mailbox For > Sent.
Highlight [Gmail] Drafts in the sidebar and select Mailbox (menu bar) > Use This Mailbox For > Drafts
Highlight [Gmail] Trash in the sidebar and select Mailbox (menu bar) > Use This Mailbox For > Trash
Highlight [Gmail] Spam in the sidebar and select Mailbox (menu bar) > Use This Mailbox For > Junk
Once properly configured, managing email from Apple Mail or the iPhone will be no different from managing emails within the Gmail web client sent, drafts, trash, and junk properly sorted between your various email clients and web interface.
Sens yourself a test mail and make a Saved folder in Apple Mail, then move that message to the new Saved folder (or whatever you want to call it. Then go back to the Gmail web page and you will see that Gmail has created a "label" for you with the same name as the saved mail folder you made in Apple Mail and if you click that label you will see your test message. Presto.
Now send a message to everybody in your address book with your new address. Just send one message with all their names in the BCC field.
Okay that cover GMail and your Apple Mail client. Now for the iPhone.
We are going to put everything in your iMac address book and sync that to GMail.... then that to your iPhone... all over the air.
So make sure your iMac address book has all the numbers/info in it you want because this is going to wipe your iPhone address bookk and replace it with the address book info.
So after updating your iMac address book go to prefs in the address book and click accounts tab.... check off sync with Google and enter your user name and PW. This will sync your address book to Gmail. Afterwards go to GMail web page and you will see all your address entries under Gmail "contacts."
Now go to iCal on your iMac and in the prefs go to Account tab and click the plus at bottom left and add Google to sync your iCal to Google Calendar. You will see a new entry in iCal for your Google Calendar. Put a test appt in there and go to Google Calendar (in Gmail web page you will see the calendar link at the top left) and you will see your appt.
Okay so now we have mail, address, appts, alll syncing to Google. Now we will add all this to your iPhone.
Fire up your iPhone and follow the instructions on this page. That will add push email, address, and appts to your iPhone.
http://www.google.com/support/mobile/bin/answer.py?answer=138740&topic=14252
Once it is all working... go to iPhone settings/Mail,Contacts,Calendars... and then the Fetch new data setting... it should be set to push=on... scroll down to advanced and leave contacts/calendar to push... but change Mail to manual. This way appointments and address book will automatically update over the air, but mail will only update when you start iPhone mail and manually check for mail. You can leave mail push on if you want but it runs down the iPhone battery faster.
Once all this is done you will have calendar, address, mail sync between Gmail, iMac and iPhone with no intervention on your part. You can fire up any browser at any desk and go to your Gmail account and check mail/address/calendar and add appts etc... those will all be synced back to your iMac and iPhone. All free.
If you want you can keep your old email for awhile and just fwd it to your Gmail address.
=========================