nozturk, You have two correct choices with similar meanings: Please be assured that ..... (please) be satisfied about the truth or certainty of something; confident or Please rest assured that......(please) remain confident about the truth or certainty of something Please be rest assured that .....mixing the two together is a mistake

I don't know of 'be rest assured' ,it is just 'rest assured' ,which means be certain.

"Please be rest assured" is not possible, as "be" and "rest" are both verbs. They do not follow each other in that fashion or context. "Please rest assured" is the proper phrase.

rest assured....correct be rest assured...wrong