Hi Prasad,
Generally, messages are put into Sent Items folder means that the email has hit the outgoing mail server successfully.
It looks you are using Outlook 2010, correct?
Does this issue only happen when an attachments is added? Does it happen to normal emails without attachments?
Was the user trying to send emails to internal addresses or external addressed?
Besides, please also try log into OWA and then test to see whether this issue continues.
Regards,
Steve Fan
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