This is one of the most-asked email marketing interview questions faced by an interviewee. Yet, this is one tricky question to answer as there is no specific time for sending a mail so that you can raze the bounce rate.
Depending upon the type of product you have you should adjust the sending time of your mail. But that is not the specific answer the interviewer is looking for. To be on the safe side say Wednesday afternoons.
Since every individual has own set of preferences to come online, you should attempt a different time slot according to the preference of your potential clients.