I think it’ll be an issue for everyone.
From my limited understanding of this, it seems it might be a mail server reputation issue.
BC had a more established email server whereas Treepl’s might be fairly new???
I ran treepl.co and the mail server IP 34.217.169.117 through a general blacklist scan and it came out pretty clean. However, one undeliverable bounce message I received from a clients site suggested that proofpoint was blocking this IP due to a sender reputation issue:
https://ipcheck.proofpoint.com/?ip=34.217.169.117
I’m guessing this will only affect recipients using certain email providers (one’s that use proofpoint or other vendors with similar email security configurations)