The error listed reflects an invalid character in the 'sender' address.
To simplify a web search, use 550 5.1.0 bounceback code, as that's what the actual code is in the message.
It's not clear whether the [ and ] brackets are incorrectly listed in the sender address, but, as we all know, those shouldn't be there.
We have had instances where list email addresses were uploaded from .csv file converted to .txt and the .csv table specifications added special characters within the .txt file, generating similar bouncebacks. The addresses needed to be manually removed and then re-added to correct the issue.
Regards,
Suzi
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