Unread mail message turns to read

Messages in Inbox show correctly as Unread (BOLD) or Read (NOT BOLD). However, after deleting a message in the Inbox, the message immediately underneath the deleted message changes from BOLD (Unread) to NOT BOLD (Read). This problem always occurs, without any regard to how long the messages have been sitting in the Inbox.
Please let me know how to fix this problem.
Thank you, Lou