old mails appear but their content cannot be read.
can you help me.
If you have only (just added) an eg: IMAP mail account, sometimes old email can take a little time to read depending on how many emails you had in your mailbox. eM Client has to sync / cache the email headers & body of the emails from the server.
If you are using IMAP it may also be that the message cache has become corrupt. Right-click on the folder that contains the messages (for Gmail right-click on All Mail) and choose Properties > Repair. That will resync the message cache with the server and the message bodies will appear.