iPhone seems to be caching passwords or credentials, how to remove them?
Your IT department is right. Your iPhone applications are caching account and password informations, even when the last typed value is plain wrong.

Your IT department wasn't helpful enough.
They should have also told you that your company PC, and most notably applications on it could also be caching account and password information.

All applications requiring an authentication are caching accounts and passwords.

In both cases (iPhone applications, and PC applications) your IT department should know which one is causing your account locking (they have the originating IP address and the server rejecting your account).
They should provide you this key information to avoid you to destroy your own configuration work.

I am sorry, all the attempts you described in your question look like the use of a baseball bat to hunt a mosquito in a porcelain store.

1st thing to do:
ask your IT department where are coming your account connection attempts with an incorrect password.
If they are able to provide you a clear answer to this easy question,
then ask them the 2nd one:
ask them with which protocol was this account connection attempted.

This information will enable you to focus on the equipment (either iPhone or PC) and application to fix (either E-mail client of web browser or Wi-Fi access).

Then what you will have to do is:


Erase just the registered password within the culprit application. On the iPhone you might have to erase the account in the case of Mail or Thunderbird application.
Connect with this culprit equipment and make an explicit connection with the culprit application, and make sure you enter your correct password. This will store this one in the application cache,
and will stop the use of the wrong one which was previously stored.




If anything in this draft answer is unclear or isn't directly adressing the original problem,
please feel free to improve the question or my answer.