Usually we need a few recommendation letters when we apply for a job or academic positions. Im wondering if there is any online server to make it easier on my referees so that they only write once their letters and later I only share the link with the companies or who asks for a recommendation. Any suggestion is welcome.
In mathematics there is, and it is used by most research-oriented departments for hiring: MathJobs. Hiring departments pay (i.e., not candidates) and candidates indeed cannot view the recommendation letters.
It is extremely efficient, and in my opinion this system deserves to be more widely imitated. Note however that this is used at the option of the hiring department, and not the job candidate. So in particular you can't use this system, except for positions advertised via this site.
In your situation you might be able to use Interfolio, but I'd check with prospective employers (or your advisor) before using this.
Yes, it's called: https://www.interfolio.com/
And you can't see the letter, as they validate any request. Works quite good. It's not free though.
Authentication is an important aspect of references and letters of recommendation. If the letters were uploaded to a third party site what assurances would the recipient have that they were genuine? If they just had to click on a link on some site there is a great deal of potential for fraud. Anyone could upload anything purporting to be written by anyone.
People will just start writing and uploading their own letters.
Most reputable employers and universities would expect a letter to come direct from the writer or to be uploaded to their own systems using authentication supplied to the author.
That is a good idea, however, such letters won't be specifically addressed to (for instance) University X or Prof. Y. They would be generic letters. Most institutions prefer letters addressed to them (hence, they are using the current system).
Im wondering if there is any online server to make it easier on my referees so that they only write once their letters and later I only share the link with the companies or who asks for a recommendation.
I was recently in a position of asking multiple professors for letters of recommendation for applying to grad school. I had about ~8 applications, so ~8 letters from each professor. All of them were happy to send them through the standard protocol (through the application website). I think the case for applying for a job or any other academic position should be similar in this respect; your referees will understand that you are applying to multiple places and will not be bothered by the request for multiple letters.
To be on the safer side, you should explicitly tell them roughly how many places you will be applying to, in case they would like to slightly tailor the letter for each application.
Lastly, follow the relevant tips in the answer to "How do I effectively solicit a strong letter of recommendation from a professor?".
From your comments:
that is the open point. is it so critical that I dont see? I must not be able to edit though
This depends on the referee. Some might want to keep the letter confidential while others wouldn't mind freely sharing the letter with you. Here is one case where the student's professor did not write a letter since the right to view the letter was not waived.
Then how to deal with asking the referees to send multiple recoms to different email addresses? I feel rather embarrassed.
Don't feel embarrassed. Just ask.
As has been pointed out in another answer, the authenticity of the letter partially (if not fully) rests on the email address it came from. The letter is of little use to the university/company that you are applying if they cannot be confident of its authenticity.