Sending eMail through Outlook using Redemption

We provide several applications that produce high volumns of email (paystubs, W2s, AR statements, etc. - never SPAM) by using Redemption to send the email to Outlook. It works very well except when the user is on an Exchange server and uses Outlook 365.  In that case sometimes, not always, some of the email generated by our program doesn't arrive at the Outlook Outbox. 

Has one else encountered this problem, and if so have you found a solution/work around?

Thanks,

Sal