SORBS is a blocklist, which is a little different than a blocklist. They aim to put all marketing email shared IP’s on their blocklist. HubSpot sends marketing email through shared IP’s so they are accomplishing their aim by putting us on their blocklist.
Fortunately, SORBS listings cause virtually zero bounces (quarantines, spam folder placement, etc…). For example the SORBS bounce rate last month was 0.0011% of sends on affected IPs in August. This is low because the recipients that use SORBS service to filter spam are also agreeing to block third-party-sent marketing email, which many IT security professionals deem to be too aggressive.
They are rare, but if you are actually seeing bounces that reference SORBS, in general my advice would be to respect the security systems that the recipient put in place to protect their inbox. However, in the case that the recipient themselves expresses to your organization that they want your email, then you should--and I’m sorry for saying this--have them allowlist your portal’s marketing IP’s. They would need to make an exception for our marketing email as it is against their general email policy.