That auto-response from BW Legal is not a genuine email bounce. A true "bounce" (either hard or soft) would be generated by a mail server, not by a noreply autoresponder. This message confirms that the email was received by their system, but they are choosing not to acknowledge or process it, which is an entirely different matter.
The key phrases:
"Your email has not been received"
"It is being returned as undelivered"
are misleading. If the message were undelivered, your email client or server would have issued a Delivery Status Notification (DSN) or Non-Delivery Report (NDR). Instead, this is just a standard template intended to deter direct email communication.
This practice raises legitimate concerns about transparency and accessibility, especially for debt-related matters where secure and provable communication is vital. There is also be an argument that such conduct breaches the SRA Standards and Regulations, including the Code of Conduct for Solicitors.
Try resending to disputeresolution@bwlegal.co.uk instead, making sure you CC in yourself.