BTW in the case it's confirmed that I'll not get my rep back if no bounty is awarded, I'd be curious to get some explanation about this rule, which seems pretty hard and not really fair.

You were told the reputation could not and would not be returned. When you clicked "start a bounty" this is what was displayed:

offer 50 of my own reputation for an answer to this question

(this reputation is deducted immediately and cannot be refunded)

You can start a total of 3 simultaneous bounties learn more

Since you had never issued a bounty before, I presume you clicked on the "learn more" link, after reading that reputation could not be refunded. Here is the fine print you agreed to:

A bounty is a special reputation award given to answers. It is funded by the personal reputation of the user who offers it, and is non-refundable. If you see a question that has not gotten a satisfactory answer, a bounty may help attract more attention and more answers. Slice off anywhere from +50 to +500 of your own hard-earned reputation, and attach it to any question as a bounty. You do not need to be the asker of the question to offer a bounty on it.

• All bounties are paid for up front and non-refundable under any circumstances.
• If your new reputation brings you below the requirement for any privileges, you will lose access to those privileges.
• Users may only have three active bounties at any given time.
• Questions may only have one active question bounty at any given time.
• To avoid overly promotional bounties, if you are offering a bounty on a question that you have already posted an answer to, your minimum spend is 100 reputation (not 50).
• Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on).
• You may not cancel a bounty once it has been started.

The system attempted to educated you on exactly how the bounty system worked.

Help Center > Reputation & Moderation

it's like spending money for nobody's benefit, and without getting any service we paid for!

You are not buying an answer to your question, the bounty is for the placement of your question, in the list featured questions.

What happens when I start a bounty?

The bountied question will appear with a special indicator in all question lists, and it will also be visible on the homepage Featured tab for 7 days. Part of what you’re “paying for” with the bounty is to get additional attention for your question, over and beyond what a normal question gets. In that regard, a bounty does not guarantee a response, and reputation refunds are not available if no answers are received as a result of the bounty.

Help Center > Reputation & Moderation

Which seems pretty hard and not really fair

I am sure you have realized by now that life isn't always fair. The good news in this case, you are getting the extra attention to your question, since your question is featured listed of questions.

I assigned a bounty to my question and I still have not get any answer while bounty-end approaches, so I wondered what will happen if nothing changes.

This is an indication a problem might exist with your question. You might consider decreasing the scope of your question. I can tell you that it will be tough to find anyone with knowlege on Windows Live Mail's internal storage structure unless it's already documented by Microsoft.

So I wondered what will happen if nothing changes.

As I pointed out, the reputation was used to pay the StackExchange Question Trolls, for them to place your question on the feature question list.