I managed to reproduce your issue.
Firstly, could you please, confirm that you have configured Fiddler to work in buffering mode (the default) which buffers the response before sending it to the client and also that you have enabled the decryption of the HTTPS traffic.
If this is the case, it seems that the server for the mail client is streaming the response and Fiddler by default does not forward it to the client until it receives it all. Hence, the mail client does not receive the response.
The solution is to enable the Streaming Mode by clicking on the 'Stream' button on the toolbar. This makes Fiddler forward any response data to the client immediately.
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