Hard to guess what you might be using, and until that is known removing it is tricky. Two possibilities to try work out what it is:
Check your Google authorisations - Have a look at the application listed here, particularly anything listed as having access to Gmail.
Check your browser plugins - a number of services work via a plugin in your browser, so have a look at what is listed there. Ctrl+Shift+A in Firefox, or via Wrench → Tools → Extensions in Chrome.
before i remember that this feature has 30 days expire date.
Did it message it to you, when config it?
Another guess is that your server config for POP3 clients that if they refresh more than for example 4 times, then block it, and when you reconfigure another IP in Gmail check that.
Best Answer
If you go to Google Accounts - Authorized Access to User Account, you can see a list of applications that have access to your Google account. You can revoke access for each individual one.