I've been battling with this issue too. I have more information over at stackoverflow - https://stackoverflow.com/questions/66012056/xcode-server-bot-not-sending-email-reports?noredirect=1#comment117216210_66012056
I think i've narrowed it down to a permission issue which can't be fixed as in Big Sur the root is read only.
I've found that Xcode post scripts can in fact run postman directly, so I will be posting an update later with how I got it working using postman directly.
Topic:
Developer Tools & Services
SubTopic:
Xcode
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