Its a out of box feature and you cant disable this feature. You can find-out details in Salesforce help and training. Just copying all the details from there for your reference..

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Due to security concerns, salesforce.com can not disable verification of email address changes without written consent from one of the salesforce.com System Administrators within your organization.

Please fax a consent letter on company letterhead, signed by the salesforce.com System Administrator stating the current situation:

1. The case number the request is associated with.

2. A specific request to disable verification of email address changes.

3. A list of the email domains which are allowed for user email addresses. These must be domains that your organization owns.

4. If more than one organization requires email address change confirmation to be disabled, please provide a valid username for each separate organization

Please fax the letter to +1� 415-592-3583 � the attention of Technical Support, and email the representative you are working with to ensure the receipt of your fax. We will process the request upon receipt of the fax.
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