mail stuck in Outbox

[Chris]

A user who uses RPC/https all the time on his laptop even in the office. In
general he has no problem to send/receive emails. But when he tried to send
a Word document from File, Send, Recipient (as attachment) it looks like the
email was sent (nothing in his Outbox under Outlook) but nothing in Sent
items. The recipient never received it. If accessing his mailbox via OWA
and MAPI profile (no cache mode) you will see the mail is stuck in the
Outbox, with envelop closed. To send 1) using MAPI open the mail and send.
It works every time. 2) using OWA it might get sent but no always and wth
delay.

He has no problem to send email using Outlook with Word attachment. I tried
the MAPI connection on another computer, not his laptop. I also was able to
send via Word using MAPI connection. I didn't try with Excel but seems to me
that the mailbox on the server has received the outbound email (othewise, I
won't see it from another MAPI conneciton) but can't send. Also, the outlook
client didn't get show the mail was stuck in Outbox.

any idea and how to fix it?

Thanks in advance.