How to trigger a rules action only for a specific value of a user profile field? I have created a Rules component which contains an action to send an mail.
And I have a rule created with the scheduler option. In this rule, I have set one condition like so:

if site:current-user field has value equals to something
then mail should be triggered,
otherwise it should not.

But it is sending an eMail in both cases. Any idea why?
Make sure to add a Rules condition Entity has field (related to your additional site:current-user-field in your Rules condition). And make sure to add that Entity has field condition BEFORE the condition you already have.

That's also what is mentioned in the Rules UI when you're adding a "data comparison" condition: somewhere it says "The data selector helps you drill down into the data available to Rules. To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type')".

For a video tutorial that illustrates the importance of this Entity has field condition, refer to Data types and data selection, especially what is shown between about 13:30 and 17:30 in it.