You are trying to send a mail with huge attachment and mails seems to be stuck in Outbox with the message "Outlook is transmitting the message". You try to delete the mail and you can’t.
There is a simple way to to clear your Outbox.
1. Make your Outlook offline (click File –> Work Offline)
2. Move or delete the mail from Outbox
3. Bring your Outlook to Online mode (click File –> Work Offline)
If lots of user complains about mails stuck in Outbox, check the Exchange server for any issues and also check your Global Catalog server.