Hi @Julia Inclan Regalado , I understand that you have an issue where message cannot be saved or sent because it no longer exists in Outlook on the web, can you kindly let me know if you are sending the email to contact lists?
From my research this issue may be related to events in service health, admins can view current information and updates on Service health dashboard in the Office 365 admin center (Service health - Microsoft 365 admin center).
Affected users can try manually adding addresses to their affected personal contact lists as a temporary workaround. Additionally, you may want to try sending the email in private mode.
If this issue persists, we recommend that you follow the Service Health dashboard in the Office 365 admin center for the latest information and updates.
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