User getting "message cannot be sent"

Hi @Stefan 

Thank you for posting your question in the Microsoft Q&A forum. 

We understand you're encountering the 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 typically appears when replying from a Group mailbox in Outlook on the web (OWA). 

In most cases, this message shows up when the reply draft is linked to a conversation item in the Group mailbox that Outlook can no longer access. This may happen if the original item was deleted, moved, or affected by retention policies. It can also occur when attempting to send as the Group without the appropriate delegate permissions. 

After reviewing similar situations, we’ve found that many users have experienced the same challenge and discovered helpful workarounds. Before applying those, we recommend checking a few key areas to ensure your configuration is set up correctly. 

1.Confirm delegate permissions on the group 
Members can always reply from their personal inbox copies using their own identity. However, to send a response from the Group mailbox as the Group, you must assign either Send As or Send on behalf permissions to the user.
Please follow this article Allow members to send as or send on behalf of a group | Microsoft Learn.
You can verify existing permissions with these cmdlets: 

Get-UnifiedGroup -Identity "<groupname>" | fl DisplayName,PrimarySmtpAddress,AccessType 
Get-UnifiedGroupLinks -Identity "<groupname>" -LinkType Members 

These confirm the Group type and list its members. 
Please note that Get‑RecipientPermission returns entries only if Send As already exists, it doesn’t show Send on behalf. And Get‑DistributionGroupMember fails because a Microsoft 365 Group isn’t a classic distribution group. You should use the Unified Group cmdlets above. 

2.Review Retention Policies
If you reply to older Group threads, a Purview retention policy targeting Microsoft 365 Groups can delete or move the original item while the draft is open. Please review retention for groups and adjust if needed:  
Go to Microsoft Purview > Data lifecycle management > Policies > inspect any policy scoped to Microsoft 365 Groups. If you’re using Delete only or Retain then delete with short durations, replies to old posts can fail when the source item is removed.  
You can read more at Learn about retention for Exchange | Microsoft Learn.

3.Ensure group visibility 
Groups hidden from Exchange clients or address lists may behave unpredictably in OWA. To ensure visibility: 

Set-UnifiedGroup -Identity "Group Display Name" -HiddenFromExchangeClientsEnabled:$false -HiddenFromAddressListsEnabled:$false  

Reference: Set-UnifiedGroup (ExchangePowerShell) | Microsoft Learn. 

4.Test with a new conversation 
You may try sending a new conversation from the Group in OWA. 
-If a new conversation sends successfully but replies to existing threads fail, retention or deleted item linkage is the likely cause.
-After updating permissions, test replying with the From field set to the Group to confirm Send as or Send on behalf is functioning. 

5.If the issue still persists, you may try clearing cached address for the Group in OWA, as recommended by other users in the community.

-Start a new email. 
-Type a few letters of the group name. 
-When Outlook auto-suggests the address, click the (x) to remove it from cache. 
-Manually retype the full group email address and try sending again. 

References: 
-Replying to emails from M365 Outlook group - Microsoft Q&A. 
-How to fix "This message can't be sent because it no longer exists. It can only be discarded. Make … 

I hope this information is helpful. 

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