Typically this may happen due to:
1. You may have attached a file to the body of your email in Outlook. To avoid this issue, we always recommend embedding your attachments. To do this, take a look at this article on how to embed attachments.
2. There may be a limit on the number of emails that can be sent from your Outlook.
Please note that when this issue occurs, the sender and the senders Customer Success Manager will be notified via our alert system. If this happens they will reach out to you or alternatively please feel free to reach out.