A mailroom is a place where mail is sorted. They're usually very hectic. The mail clerk is a person sorting the mail. Many larger businesses which receive lots of mail have a mailroom where clerks organize the mail by department and then deliver it.
Thank you. Actually my guess at this word is just close a second to yours. However, when I wanted to double check on it, I just couldn't find it on some dictionaries, even wiki. Thanks for help.