email does not reach the mailbox

Hi, i have user complaining to me that he all his email does not reach his mailbox after 7pm almost everyday…..the mails will only sent to his mailbox the nextday morning as he check the mail sent date it shows that it is supposed sent to him on yesterday night but it din reach his mailbox.

So may i know whats wrong with it?? the server is running on windows 2000 with exchange 2003 SP2