Based on your request, you are unable to add a note directly to an email. Here are several options that you can consider as a workaround.
Option 1: Reply and send the original email to yourself with your notes. This way, you can still Snooze your message along with the original email. Just make sure that “Grouped by conversation” is enabled.
Options 2: I personally do not use the snooze feature as a reminder to complete a task. I prefer to add the email to the TO-DO task list.
Right-click on the email | Select Advanced actions | Create Task
The To-Do allows me to:
1) Create a custom task list (Nov2do) or add the email to an existing task list
2) Add notes/next steps and track open / close statuses
3) Set reminders and due dates
4) Open the original email in Outlook
Then you can add your Task list to Outlook’s Calendar Board. Here is a video that highlights the Calendar Board.
Option 3: Send the email to OneNote
I am also a huge fan of OneNote. You can send an email directly to OneNote and include any notes that you like. However, you don’t get a reminder. And, if you are not familiar with OneNote, a learning curve is needed but nothing overwhelming.
I hope you found these workarounds helpful. You now have options.
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