All the sent mails are present in the outbox folder and not moved to sent items even if the mail is delivered

Dear Mirosoft,

I have the below issue for about 100 users in a site from a different databases and not for all users also.

"All the sent mails are present in the outbox folder and not moved to sent items even if the mail is delivered"

The problem occurs only when i had PST and make the pst as the default.

I have tried on machine where antivirus is not installed.

Thanks,

MDSB