Hi @Bk Santiago ,
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When sending an email using SMTP [smtp.office365.com], if you set up your own Message-ID, it may be replaced by a new one and moved to X-Microsoft-Original-Message-ID. This can cause threading issues in all recipients on the mailing list that you are sending to.
One possible solution is to visit the Microsoft Exchange admin center and check the box “Turn on sending from aliases” under “Mail Flow”. This should allow you to send and receive from an alias address without Exchange resolving it to the primary alias before delivery.
Related thread and details please refer to: https://learn.microsoft.com/en-us/answers/questions/133111/alias-email-address-getting-replaced-by-primary-em
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