[exim] A cryptic clue

[exim...@riotm.co.uk]

I'm not green with mail servers, but this one is new to me:

550 Sender address belongs to a non-trusted organisation [macl]

I'm told it's Exim but I'm not sure of the cause. It is given at the
rcpt to stage and I'm suspecting some kind of acl from the message.

Can anyone suggest a reason or further reading?


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