Is it right to write " Dear Sir" in a formal letter if we don't know the addressee even if he is in lower rank than the sender ? Such as Port's director writes to a company .
thanks in advance
Well it is right. You are sending the letter to somebody who you don't know, just like you said, and you it would be polite to address it Dear Sir. Besides, since you don't specifically say who the letter should go to, it could end up in hands of higher rank person.
Note: I am not a teacher.