If you say "sends" then it means the action already exists. However, the action is (in this case) requested, so it makes no sense to ask for an action that already exists. You use the base verb for situations like this: "Mr. Hawkins requests that someone send the data by fax immediately." It doesn't matter whether you have he/she/it or not. Look up "present simple subjunctive" and you'll be able to find out more about this rule.

If it had been "sends" it would be implying that Mr Hawkins was only specifying how he wishes the data to be transferred, in other words, he wishes it to be sent by fax at sometime in the future. But because the sentence ends with "immediately" using "sends" would not make sense. If the word "immediately" was not in the sentence, it would have been fine to use "sends" In the example above only the word "send" makes sense.