The bounce-back message says the sender's IP address is blocked. This means that your customer's email has been blocked by the global company "Senderscore" and your client will be having trouble contacting other emails too.
Many users with emails have a Microsoft IP address connected to their email account which is right now being blocked by hundreds of different domain names. You can see this Here
This bounce-back error message may look like the example below:
Decision Engine classified the mail item was rejected because of IP Block (from outbound normal IP pools) -> 554 5.7.1 ACL dns_rbl; Client host [40.107.108.188] blocked using sa-ip4tset.blagr.emailsrvr.com=127.22.0.2 Senderscore. Please visit https://senderscore.org/rtbl/ for more information on why this message could not be delivered (G31)
Recommended actions:
You can let your customer know that they need to request SenderScore to unblock their IP Address: https://senderscore.org/act/blocklist-remover/
It is also recommended to whitelist your customer's email and to get your customers to whitelist your email to their safe contact: How do I safelist an IP Address in my email settings?