Custom Message-ID modified by the outlook SMTP server with the ****@****.apcprd01.prod.exchangelabs.com message-ID affecting 3000+ Microsoft email account users.

Question

Custom Message-ID modified by the outlook SMTP server with the @.apcprd01.prod.exchangelabs.com message-ID affecting 3000+ Microsoft email account users.

I am sending my mail using PHPmailer using SMTP (Outlook SMTP) and i also generating custom Message-ID.

But that custom message-id is attached to X-Microsoft-Original-Message-ID header and the Message-ID header is modified with the new message-id like <*@.apcprd01.prod.exchangelabs.com> and because of this unable to send the mail in the same thread and that is the biggest problem of our customer.

Note:

If i am using Gmail SMTP (smtp.gmail.com) and sending emails then custom Message-ID is sent as per what i set.

But when i am sending emails using Outlook SMTP (smtp-mail.outlook.com), then Message-ID is updated to some string (<***@.apcprd01.prod.exchangelabs.com>)

Hope will get the solution faster and so there will be no need to switch those users to other ESPs.

Thanks and Regards