When an email is sent from Salesforce, where both the sender and recipient share the same email domain and their mailboxes are hosted in Outlook (Exchange Online) with Direct Send disabled, the message is rejected.
The user who triggered the email receives a bounce-back notification (or) flagged as a Permanent failure in Salesforce email logs with the following error :
< 550 5.7.68 TenantInboundAttribution; Direct Send not allowed for this organization from unauthorized sources >
Direct Send is a Microsoft Office 365 feature that controls how email is routed.
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