https://check.spamhaus.org/faqs/?id=PBL_what_is_it
What is the PBL?
The Policy Blocklist (PBL) is a list of IP space that should not be sending email direct to MX: often these are IP ranges assigned by ISPs to broadband or dial-up customers, but the PBL does include other types of IP space. Any IP space that should not be sending email directly to the Internet should be listed in PBL.
PBL listings do not prevent the sending of email unless the user’s email program is not authenticating correctly when connecting to their ISP or company’s mail server. Possible causes for this happening suddenly: changing email programs or changing settings on the existing one, forgetting to turn “SMTP Authentication” on, or switching “SMTP Authentication” off.
https://www.spamhaus.org/faq/section/Spamhaus%2520PBL#197
Enabling SMTP Authentication
SMTP Authentication (SMTP AUTH) is required when sending email via most major ISP mail servers and corporate mail servers. SMTP Authentication is a username+password system which ensures only authorised senders (i.e: the ISP's customers) use the outgoing email server.
If SMTP Authentication is not enabled in the user's email program or App (e.g: Outlook, Apple Mail, Thunderbird, etc.), or if the SMTP Authentication process fails (such as due to a wrong or mistyped username or password) most ISP email servers will not accept outbound email from the connection - and the reason why is very simple: Outbound email servers need to know the connections they accept are from authorised users/customers only.
Unfortunately, mail servers are not very good at explaining why they have refused a connection, and because the Spamhaus PBL is used by mail servers to determine what to do with 'unauthenticated' connections, when Authentication fails, the error/reject messages (over which we have no control) tend to say "Blah blah, blocked... blah blah ...Spamhaus" without explaining that all the user actually needs to do is turn SMTP Authentication on.
Things to double check if enabling SMTP Authentication does not work as expected:
Is the information provided for the outgoing email server hostname, username, and password correct?
Is SMTP Authentication working correctly on the email server? This may be a question for your ISP.
Is the port number in use correct? For SMTP Authentication to function correctly, the port should be 587 or 465, not 25.
For help with configuring specific email software for SMTP Authentication, please consult your ISP user manuals or help webpages. Most companies publish a user portal with instructions that can be easily found with a websearch.
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