webmail issue

Been doing everything according to http://drupal.org/node/369604
Mail API:
Protocol:IMAPWU
1. Server Hostname: (my IP Address)
2. Server Port: 143
3. Encryption: none
4. Domain: (my IP Address)
5. Connection options: {removed all settings written there, means field is blanked}
6. Login Suffix: @(my IP Address)

Webmail plus:
1. Mail Connection:(My IP Address)
2. Mail delivery:
Mail Connection:SMTP
SMTP host: localhost
SMTP port:25
Encryption:None

I haven't bothered going any further because after i set the Domain in Mail API to (my IP address) then my Mail Connection is automatically changed to (My IP Address) in Webmail Plus, i get the following error:
Fatal error: Call to undefined function imap_open() in /drupal/sites/all/modules/mail_api/contrib/imapwu_api/imapwu_api.module on line 73
once i remove my IP address from the domain the error is gone.....