O365 server blocked by SORBS

Question

We're struggling sending emails from Office 365 to multiple domains, recieving an error message like the following:

SMTP error from remote mail server after RCPT TO::
554 5.7.1 Service unavailable; Client host [IP address] blocked using dnsbl.sorbs.net; Currently Sending Spam See:
http://www.sorbs.net/lookup.shtml?[IP address]

This has been an issue since february and does not seem to be going away by itself. Do you have any suggestions?

Answer 1

Open a ticket with Office 365. If you are sending mail through them and getting blocked, that means your messages are being sent out from the 365 High Risk Pool:
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/high-risk-delivery-pool-for-outbound-messages?view=o365-worldwide#high-risk-delivery-pool

The high risk delivery pool is a separate IP address pool for outbound email that's only used to send "low quality" messages (for example, spam and backscatter). Using the high risk delivery pool helps prevent the normal IP address pool for outbound email from sending spam. The normal IP address pool for outbound email maintains the reputation sending "high quality" messages, which reduces the likelihood that these IP address will appear on IP block lists.

The very real possibility that IP addresses in the high-risk delivery pool will be placed on IP block lists remains, but this is by design. Delivery to the intended recipients isn't guaranteed, because many email organizations won't accept messages from the high risk delivery pool.