When someone tries to send you an email and they received a bounce back error message from Mailer-Daemon with the following error message in it, "SMTP error from remote mail server after RCPT TO <your email address> 554 5.7.1 Service unavailable. Client host [sender's IP address] blocked using SIP invaluement local."
This means that the sender's domain name or IP address is on a blacklist. As a precautionary measure, Paubox's spam filtering system blocked this sender's email to you to prevent any malicious emails from coming through.
To verify that the sender's domain name is on a blacklist, you can use MX Toolbox, a very reliable source.
If you would still like to receive the emails from the sender, despite their blacklist status, please notify us by submitting a ticket request for the sender's IP address to be allowed on our system.
Please include the sender's IP address, or a screenshot of the error message in your ticket request.