Email Return error

Am i right in thinking that this email error is a result of the sending server refusing to accept bouncd emails, or could something else also cause this error message?

>550-Callback setup failed while verifying <name@removed.byme>
>550-(result of an earlier callout reused).
>550-The initial connection, or a HELO or MAIL FROM:<> command was
>550-rejected. Refusing MAIL FROM:<> does not help fight spam, disregards
>550-RFC requirements, and stops you from receiving standard bounce
>550-messages. This host does not accept mail from domains whose servers
>550-refuse bounces.
>550 Sender verify failed

This above is the message that they get when they try to email a client on my server. The sender is using a large Uk based ISP to send email and they are saying they only have trouble when emailing my client, which I find hard to believe if the above error message is correct.

Thanks

 

 

 

 

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