Hi Bill,
Thank you for posting, I am here to help you. Possible reason is due to poor internet connectivity, large attachments, Outlook being in offline mode, or conflicts with security software like antivirus.
Are you using Outlook Online or Outlook Apps on Windows.
Using Outlook Online OWA follow this steps:
Clear cookies / cache of your web browser.
For more information check this article:
Outlook email stuck
https://support.microsoft.com/en-us/topic/outlook-email-stuck-5c945f98-415e-58f2-8476-b76e59dfd2e3
I hope you find this information helpful. Please let us know if you have any further questions If the answer is helpful, please click "Accept Answer".
Best regards,
Jansher