Thank you for your post and also thanks for David and Cliff's input.
Before we go further, I would like to confirm with you that:
1. Does this issue occur only when sending outbound emails to
XXXX@XXXXX.COM or the whole @btinternet.com ?
2. Do you mean this issue does NOT happen to the user who has
XXXX@XXXXX.COM and happens to all of the left users?
3. Do these users receive bounced back NDR letter from the btinternet.com
or emails to btinternet.com are being stuck in SMTP queue?
Based on my experience, If the issue did not occur before and occurs
recently without changing any settings on the SBS server, then it may be
the destination domain issue which may set certain filter to block a group
of users from your domain. Please contact the btinternet.com for more
information, if possible.
Meanwhile, to troubleshoot mail flow issue, I suggest you enable the
Message Tracking feature in Message Tracking center to track the outbound
emails and see if mesages are stuck or has been delivered to the Internet.
Please refer to the following articles for details steps:
Exchange 2003 Message Tracking and Logging
Moreover, here are two artilces contains general mail flow troubleshooting
steps, please have a look:
How to use Queue Viewer to troubleshoot mail flow issues in Exchange Server
How to troubleshoot for Exchange Server 2003 transport issues
Hope this helps. Also, if you have any questions or concerns, please do not
hesitate to let me know.
Thank you for your time and cooperation!
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