Hi Mark,
This is Christina from Office 365 Exchange Online Support team. After reviewing this blog, I understand currently the issue you are experiencing now is that some users cannot send email out via OWA, all outgoing mail stuck in the draft folder.
To further troubleshoot this issue, really appreciate your help to collect following information and send them to me via PM:
1. How many users meet this issue currently? Please provide me a detailed affected users list.
2. Please kindly provide me two sample screenshot of this issue for two different affected users.
Looking forward to hear from you.
Best Regards,
Christina Yi