Office 365 to CPanel connector fails validation test

I'm trying to set up an outgoing connector, but it keeps failing the validation step. The MX record is currently pointing at the CPanel host, and I don't want to change that until I get the connector going. The validation address I'm using is actually a forwarder at CPanel - there aren't any "real" mailboxes there. Nevertheless, it's a valid email address and people have no problems sending emails to that address. The issue seems to be that MS365 cannot validate the existence of the address, but I've no idea what I can do about it. The full log is below:

Resolve 'mail.vocalmix.org'
Succeed
OK - Resolved to '23.235.222.247'
Connect to '23.235.222.247' from Office 365
Succeed
OK - 22O Connection Successful
SMTP EHLO to 'mail.vocalmix.org'
Succeed
OK - 22O
Smtp TLS
Succeed
OK - Support TLS and certificate is ok

Detailed log

250-server1.harmonysite.com Hello mail-sy4aus01lp2175.outbound.protection.outlook.com [104.47.71.175]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250 HELP

--------------------------------------------------------------------------------------
email address removed for privacy reasons Failed
550 5.1.10 RESOLVER.ADR.RecipientNotFound; 
Recipient email address removed for privacy reasons not found by SMTP address lookup

Detailed log

Timestamp:3/2/2023 12:10:46 AM
EventId:RECEIVE
Source:SMTP
MessageSubject:Test email for connector validation
MessageId:<email address removed for privacy reasons>
Recipients:email address removed for privacy reasons
RecipientCount:1
RecipientStatus:
SourceContext:08DB1A72E2EEE6D5;2023-03-02T00:10:46.222Z;0
Sender:email address removed for privacy reasons
EventData:ProxyHop1:MEWP282CA0114.outlook.office365.com(2603:10c6:220:1d4::12), MimeParts:Att/Emb/MPt:0/0/1, MessageValue:MediumHigh, Replication:SYYP282MB1903, InboundTlsDetails:TLS=SP_PROT_TLS1_2_SERVER TLSCipher=CALG_AES_256 TLSKeyLength=256 TLSKeyExAlg=CALG_ECDH_EPHEM, CorrelationId:7985c28c-3a9c-4e99-ec28-08db1ab2918e, FirstForestHop:MEYP282MB1895.AUSP282.PROD.OUTLOOK.COM, FromEntity:Internet, DeliveryPriority:Normal, OriginalFromAddress:<>, AccountForest:AUSP282A001.PROD.OUTLOOK.COM


Timestamp:3/2/2023 12:10:46 AM
EventId:RECIPIENTINFO
Source:RESOLVER
MessageSubject:Test email for connector validation
MessageId:<email address removed for privacy reasons>
Recipients:email address removed for privacy reasons
RecipientCount:1
RecipientStatus:NotFound.OneOff.Resolver.CreateRecipientItems.10
SourceContext:
Sender:email address removed for privacy reasons
EventData:SenderVerdict:NotFound.OneOff.Sender.10, DeliveryPriority:Normal, OriginalFromAddress:<>, AccountForest:AUSP282A001.PROD.OUTLOOK.COM


Timestamp:3/2/2023 12:10:46 AM
EventId:BADMAIL
Source:DSN
MessageSubject:Test email for connector validation
MessageId:<email address removed for privacy reasons>
Recipients:email address removed for privacy reasons
RecipientCount:1
RecipientStatus:
SourceContext:
Sender:email address removed for privacy reasons
EventData:DeliveryPriority:Normal, OriginalFromAddress:<>, AccountForest:AUSP282A001.PROD.OUTLOOK.COM, BadmailReason:Suppress NDR of a rejected or expired DSN


Timestamp:3/2/2023 12:10:46 AM
EventId:FAIL
Source:ROUTING
MessageSubject:Test email for connector validation
MessageId:<email address removed for privacy reasons>
Recipients:email address removed for privacy reasons
RecipientCount:1
RecipientStatus:[{LED=550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient email address removed for privacy reasons not found by SMTP address lookup};{MSG=};{FQDN=};{IP=};{LRT=}]
SourceContext:
Sender:email address removed for privacy reasons
EventData:DeliveryPriority:Normal, OriginalFromAddress:<>, AccountForest:AUSP282A001.PROD.OUTLOOK.COM, ToEntity:Unknown, FromEntity:Internet, IsDsn:False

How do I get this connector set up?

Thanks in advance.