There are a wide variety of reasons why a sender might receive a rejection notice when sending an email. While the information may be rather technical, ultimately the reason for the rejection should be contained in the bounceback message, so it's important to have the full error message availible for troubleshooting.
While we’re certainly glad to review the message and advise the best course of action, email bouncebacks are almost always sending-side issues that would likely need to be resolved by the sender’s email administrators, so it’s best to have the sender bring the message to their email administrators for resolution sooner rather than later.
If the message is regarding a sender’s IP address being listed in an RBL / blacklist, their email administrators would need to address that blacklisting, or you can request that RBL checks be disabled for all mail sent to your domain. Please refer to the following article for more information on RBL checks:
What is a Real-Time Blacklist (RBL)?
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