Client host blacklisted

I've searched the forums but haven't found exactly this use-case.  I'm trying to send an email to a domain, but it's being rejected by its MTA with the message

554 5.7.1 Service unavailable; Client host [..] is blacklisted

When I visit the lookup page (included in the message) at https://www.sophos.com/en-us/threat-center/ip-lookup.aspx I get redirected to https://www.sophos.com/en-us/labs (presumably this is updating tools) but it shows a green "OK" for my IP.  I've also checked all blacklists and the IP has a good reputation.

I'm a bit stuck - I cannot send email (IP is blocked) and I cannot unblock (IP is not blocked).  Never the less, I would like to send email.  Perhaps someone has a tip on what I might be missing?  

Thanks!

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