Using outlook to send email via MAPI

I have a MAPI client application that will lauch outlook with a list of
recipients from a database. It used to work fine. But now the email will be
bounced without sending out. But if I retype the recipient email addresses
after the outlook is brought up, the email will be sent. Why my outlook does
not like the recipients assigned by the MAPI application? What can I do to
make my outlook to send the email?

I have outlook 2003 in my computer. Thanks for any help.