Unfortunately I don't know much about email servers and I don't know how I would be able to tell what the reason is, sorry.
The email I originally contacted was taxclientsupport-itch@taxidentity.com, which is the email listed in the tax interview FAQ, so I figured that it would've been updated if the address had changed.
For clarification, when I say that I contacted Itch support I mean that I then directed my original question to them instead, not that I asked for help regarding the bounced email. I tried looking but it's not a question that I can search up.