Some users of Classic Outlook are unable to send emails via Outlook.com due to a known issue. When sending or replying to emails, they receive an error message saying “This message could not be sent.” The problem occurs when the account is linked to another Exchange account as an Outlook profile. Microsoft has identified two conditions that trigger these errors: having an Exchange Online mail contact with the same SMTP address, and using the M365 Address Book.
To temporarily fix this issue, Microsoft recommends removing the M365 address book or hiding it from the GAL, creating a new classic Outlook profile for only the affected account, or sending emails through the New Outlook client or Outlook.com on the web. In recent two weeks, Microsoft has fixed other known issues affecting Classic Outlook, including crashes and synchronization errors with Gmail and Yahoo accounts.
Additionally, Microsoft is investigating bugs that cause connection issues when creating groups in certain apps and disappearing mouse pointers in some Microsoft 365 applications. Meanwhile, a new whitepaper addresses automated pentesting tools and their limitations, providing diagnostic questions for tool evaluation.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-links-classic-outlook-bug-to-email-delivery-issues