Our client received a bounce email when sending a mail thru siatyun.com.sg.

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Here's the message received:

The following message to <email> was undeliverable.

The reason for the problem:

Unknown address error 550-"SC-001 (BAYO-PAMC2-F6) Unfortunately, messages from ***.***.***.*** weren't sent. Please contact your Internet service provider since part of their network is on our block list. ...

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Can check, how this following email:

unable to received by any user account under @siatyun.com.sg

Thanks.