How to respond to an email from a group?

The behavior and error message match a known issue with replying to Microsoft 365 Group messages directly from the group mailbox in Outlook on the web or the new Outlook.

When opening a message in the group, choosing Reply all, typing a response, and sending, Outlook can show:

"Couldn't send this message"
"This message can't be sent because it no longer exists. It can only be discarded. Make sure you copy the contents of the message before you discard if you want to use them later."

This is currently under investigation. The documented workaround is:

1. In the group mailbox, open the message.
2. Use Reply instead of Reply all when responding from the group view.

Alternatively, continue using the method that already works in the described scenario:

• Receive the group conversation in the personal inbox (by following the group), and
• Reply from the personal inbox, where replying to the group succeeds.

To send new messages to the group from Outlook:

1. In new Outlook or Outlook on the web, select Groups in the navigation bar and choose the group.
2. Select New mail.
3. In To, ensure the group’s email address is present, add a subject and message, and select Send.

References:

• Can't use Reply All in email messages in Microsoft 365 groups
• Send an email to a Group in Outlook
• Send an email to a Microsoft 365 Group in Outlook for Mac