Hello, Here is the scenario, when the external domain user sent an email to particular 3 users to our domain Two user received the mail, but the one user email missed in the TO filed when we tracked in the exchange 2010 message tracking log. But the email has crossed the gateway for the missed recipients. We don’t have any clue were the email id has removed from the TO filed. User who received the email had the missed user name in the TO filed, but in the message tracking there was no log for the missed user. The user email that has missed in TO field doesn’t have any issue in the mailbox. Thanks& Regards Jeevananthan S

Are you certain that the recipient was put in the To field when the message was sent and not the Bcc field? If a user is a Bcc recipient, that doesn't show up in the headers.Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."

You can look at message tracking or SMTP protocol logs to see if your inbound mail gateway was told of the recipient. What is in the RFC822 headers (what the user sees in Outlook) is meaningless to Exchange for routing purposes. If their sending MTA didn't specify that user during the RFC821 portion of the message transmissal Exchange wouldn't know to deliver it to that user.

Hi, Besides, How about trying to send a mail from missing mailbox to the external domain then replying the mail to the user?Rowen TechNet Community Support

Hi, Any update?Rowen TechNet Community Support

Hi, That particular user had an issue in the gateway.still we are investigating why the email id has removed in the gateway. i will update the information once we done our troubleshooting part.Thanks& Regards Jeevananthan S

Hi, That particular user had an issue in the gateway.still we are investigating why the email id has removed in the gateway. i will update the information once we done our troubleshooting part.Thanks& Regards Jeevananthan S

Hi, Finally we found the root cause for the issue. The external domain had a local mailbox in our user email address, which made the routing internally before it move to the external routing. so that the email id was removed from the To filed. Thanks& Regards Jeevananthan S