Hello JaneRowe- [O365],
Thank you for posting this case in Microsoft Community!
Based on your description: Missing 'Edit Message' quick access toolbar button. I would like to cooperate with you working on this case. To clarify this case, how did you access the Quick Access Toolbar? Please understand that Edit Message command is only available for opening one existing email message to access Quick Access Toolbar. For my suggestion, you may try following workaround to fix your case step by step:
- Firstly, you may go to your Inbox folder and select to open one existing email message. On the top of your message window, you can click the downward arrow icon to access Customize Quick Access Toolbar list. And then you need to click More Commands... options.
- After that, it will pop up Outlook Options window and you need to access Choose commands from to select Commands Not in the Ribbon options. Then you can scroll down to find Edit Message command and click Add to the right Quick Access Toolbar.
- Finally, you can click OK to save this settings and see the Edit Message icon showing in your Quick Access Toolbar list.
See more information, you may check this article why can't i find the "edit message" command in the quick access toolbar in outlook 2016 as similar case reported before.
Hope the above workaround would be helpful. If you need further assistance, please feel free to let me know, I will keep assisting you.
Your understanding and patience will be appreciated.
Thank you!