These instructions apply to the iPhone, iPad and iPod Touch, and are for iOS5. Other versions of iOS should be similar
On the iPhone:
Go to Settings
Tap 'Mail, Contacts, Calendar', then 'Add Account'
Tap Other
Choose the Mail account type
Enter your Name, email address, VPOP3 password, and press Save
Choose IMAP (or POP if you have VPOP3 Basic)
Scroll down and fill in the Incoming Mail Server details
The Host Name is the the external IP address of your office network which is mapped through the router to the VPOP3 server (or the host name you have defined in your domain configuration which resolves to that address).
Determining VPOP3 server address
The User Name is the VPOP3 username
The Password should already be completed
Scroll down and fill in the Outgoing Mail Server details. The details are the same as for incoming mail.
Press Save
If you receive a prompt saying “Cannot Connect Using SSL - Do you want to try setting up the account without SSL?”, select 'yes'. See below for further details.
For general advice on using mobile devices, see Using a mobile phone with VPOP3
There are two methods to support SSL encryption in email. The current way uses the same 'port' as normal, and sends a command to start encryption, and an obsolete way uses an alternate port.
It appears that the automatic setup system for iOS mail accounts only handles the obsolete method, even though the mail client itself handles the current method as well.
So, to use SSL encryption, follow the steps below (yes, it is a bit dumb that this is necessary (other email clients handle this automatically), but talk to Apple about that):
During setup, tell your iPhone/iPad to trying setting up the account without SSL
In the Mail Account list, tap the account you have just added
Tap the line beginning with Account at the top, to edit the account settings
Tap the Outgoing Mail Server SMTP line
Tap the *Primary Server line
Select the Use SSL option
Ensure the Server Port is correct - usually either 25 or 587 as appropriate (not 465)
Tap Done, and <Account
Tap Advanced at the bottom
Select the Use SSL option
You will note that the 'Server Port' has changed to 993 now. Change that back to 143
Tap <Account and Done