This might be because your email address is being used for your company's web integration in the system such as RESTlets, webservice due to failure authentication.
To verify, you can follow the instructions below:
1. Using an Administrator role, go to Setup > User/Roles > View Login Audit Trail.
2. Click Create Saved Search button.
3. Under Criteria, under Standard subtab, select "Email Address" and on the field, enter the email address indicated on the Login Failure Notification email, then click Set button.
4. Add Date and set it from the date when the Login Failure Notifications was first and last received.
5. Under Results Tab > Add Request URI.
6. Add Detail as well on the Results tab, to view more verbose information about login attempts.
7. Add Token-based Application Name and Token-based Access Token Name to know more about which integration is failing with the authentication. This usually comes from /app/site/hosting/restlet.nl or /app/site/hosting/webservices.nl
7. Add a title of your search then click Save & Run button.
The Detail column in the search results contains more information about the login attempts, and provides information about the reason for the authentication failure.
The following messages indicate some of the most common issues:
NonceUsed — The combination of nonce and timestamp was already used by the same user.
InvalidTimestamp — The timestamp provided is different from the correct server time by more than five minutes.
InvalidSignature — The request was not signed correctly.
You may also export it by clicking Excel icon and send the file for us to confirm your checking.