DSN code 5.7.1 error in O365 emails

my user received the following error message when submitting claim from our inhouse system

eclaim@goldexxxxxx.com.sg
Your message couldn't be delivered because there is a partner connector configured that matched the message's recipient domain. The connector had either the RestrictDomainsToIPAddresses or RestrictDomainsToCertificate set.
For more information about this issue, see DSN code 5.7.1 in Exchange Online - Office 365.

PU1APC01FT038.mail.protection.outlook.com gave this error:
TenantInboundAttribution; There is a partner connector configured that matched the message's recipient domain. The connector had either the RestrictDomainsToIPAddresses or RestrictDomainsToCertificate set [PU1APC01FT038.eop-APC01.prod.protection.outlook.com]