Twice in the last two days, emails I have sent to legitimate contacts have been bounced back with

Service unavailable; Client host [212.23.3.141] blocked using backscatter.spameatingmonkey.net; listed, see http://spameatingmonkey.com/lookup/212.23.3.141

They emailed me, I replied. Then I got bounced back.

Is there something wrong with the client host, which is the same in both cases?

(In the first case I phoned her, and her response was "it must mean my mailbox is full". Is this likely considering the bounceback message?)

Baffled. Thanks for any replies.