I am concerned to the fact that how a professor whom I am asking to write an LOR, will change the name of the person every time before sending? I am applying to like 100 different persons for the position of postdoc. Does it really matter that it should be addressed appropriately (name basis)?
I am working in the interdisciplinary field of materials chemistry, nanotechnology, environmental remediation, physical and theoretical chemistry.
Not all professors customize their letters to the specific recipient. It probably helps a little (especially if the recipient is at a prestigious institution, so that they know that this letter isn't directed at a generic less-prestigious institution), but it's not that necessary.
But you definitely should. If you are applying to 100 different professors for the position of postdoc, and you send them a form letter saying "I am very interested in your work" without once mentioning what their work is, they are very likely to realize that it is a form letter, decide that you're probably not all that interested in what they're doing, and disregard your application.
So it's probably not enough to just put in their name, but you should also mention something about their field of research that interests you and explains why you would be the correct postdoc for them.
Is this a lot of work? Yes, but consider that you're competing for postdoc positions with a lot of other people who may not mind going to the effort of putting in a lot of work in order to get a job. Do you think professors want to hire lazy postdocs?
I can hardly see the need of addressing the reader in a letter of recommendation, but if your professor wants to do it, they can use a process called mail merge.
Common word processors have mail merge functionalities (e.g. MS Word).
A customized letter of recommendation shows that the writer cares about the success of the applicant. Each letter should have the name of the recipient, or if unknown, the recipient's institution, at the beginning. It should also address the specific requirements of the position.
If your professor has basic programming skills, they can do this pretty easily in any common programming language, such as C or Python.
If it is important that your letters be personalized (unlike say in mathematics where generic letters are 100% expected), I would be more concerned that the letters are not contextualized rather than them not being addressed. Letters of recommendation should describe how the applicant would fit in and contribute to the institution receiving the letter. If your reviewers expect your letters to be personalized, they may be disappointed that the content of the letter doesn't describe how you and your experience relate to their institution/research project/etc. In some fields this does not matter (math and CS in particular expect you to mail one letter to a thousand people), but it might in your field.
