I have got letter of experience from my previous employer on company's letter head, but it doesn't have a company stamp or seal upon it. Would it be considered as a valid experience letter? Or is getting it stamped mandatory?
It would be good and feel authentic if the letter is stamped.However, I don't think it is mandatory.
Plus, it varies depending on company / geography. In India, (where I work out of)as far as I know, it is required to have the experience letter printed out with a proper letterhead bearing the employer's logo and seal, signed by an authorized HR Officer.
I checked this with a HR guy I know and he tells it is not obligatory for a company to stamp the letter(We are a MNC). I guess you are fine, provided the letter carries an authorized signature.