Specific sent emails missing from conversation for user

Question

User replied to an existing conversation from a personal Exchange Online mailbox using Outlook on the web. Message was sent from the primary SMTP address (not an alias) and personal mailbox not a shared one. Recipient received the email and replied, quoting the original message. Sender sees only the grey “You replied — date/time” marker/banner in the conversation; no Sent Items copy exists anywhere in the mailbox (Sent, Deleted, Archive). Same behavior is visible in Outlook desktop. No mail flow rules beyond block/allow, no add-ins, no shared mailbox. Message Trace confirms successful delivery.

This issue happens to the user occasionally. Most replies get saved in the sent folder as usual but every now and then a single reply does not show up in the conversation other than as a banner with a time and date.

Platform: Outlook web (also checked the desktop application and cant find it there either).

Desktop version: 1.2025.1209.500

Response sent from: Outlook Web

Browser: Chrome

Sent from: Users primary SMTP

Send method: Reply all

Add-ins: None

Rules in Exchange Admin Center: No mail flow rules modifying messages (only block/allow)

Answer 1

The issue you're experiencing, where specific sent emails are missing from the conversation view in Outlook, can occur for a few reasons. Here are some potential causes and troubleshooting steps:

1. Outlook Configuration: Ensure that the setting to save copies of messages in the Sent Items folder is enabled. This can sometimes be controlled by Group Policy settings, which may inadvertently prevent messages from being saved.
2. Mailbox Folder Limitations: If the mailbox has exceeded a certain number of folders or if there are issues with folder visibility, it might affect how messages are displayed or saved. For instance, if the mailbox contains too many folders, it could lead to some messages not appearing as expected.
3. Cached Exchange Mode: If you're using Cached Exchange Mode, there might be issues with the local .ost file that could cause messages to not sync properly. You may want to try disabling Cached Exchange Mode temporarily to see if that resolves the issue.
4. Browser Issues: Since you are using Outlook on the web via Chrome, ensure that your browser is up to date. Sometimes, browser extensions or settings can interfere with how web applications function.
5. Intermittent Issues: Since you mentioned that this happens occasionally, it could be a transient issue with the Exchange server or Outlook services. Monitoring the situation and checking for any patterns might help identify if there are specific times or conditions under which this occurs.

If the problem persists, consider reaching out to Microsoft support for further assistance, as they can provide more detailed insights based on your account and configuration.

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References:

• Email sent using Outlook aren't saved to the Sent Items folder
• Some folders are not visible in Outlook on the web