@Nik G,
By checking MTA logs,
This error is shown in the case of the O365 account.
Fork/Split: Message ID [765CD31C7B904D45A5C7341920035017.MAI] requeued/split into 1 messages for delivery to target domains.
Queued: Message ID [765CD31C7B904D45A5C7341920035017.MAI] queued for delivery to the domain [(ourdomain).com] at the SMTP Outbound Queue with a new Message ID of [765CD31C7B904D45A5C7341920035017.MAI]
Debug delivery information for Message ID [765CD31C7B904D45A5C7341920035017.MAI]
10/21/21 17:48:05 ME-I0018: [4B29353A938E432D97E7B9C50AF5C528.MAI] Outbound message from ([SMTP:no-reply@(ourdomain).com]) requeued as [765CD31C7B904D45A5C7341920035017.MAI] to the target domain [(ourdomain).com]
10/21/21 17:48:05 ME-I0026: [765CD31C7B904D45A5C7341920035017.MAI] Sending message
10/21/21 17:48:05 ME-IXXXX: [765CD31C7B904D45A5C7341920035017.MAI] DNS resolved to the following record: IP Address=104.47.101.36, Family=2, Type=1, Protocol=6
10/21/21 17:48:26 ME-E0038: [765CD31C7B904D45A5C7341920035017.MAI] Communications Error: Socket connection to (ourdomain)-com.mail.protection.outlook.com failed (error 10060). The host was either not contactable or it rejected your connection. Socket Family = 2; Host=104.47.101.36; Port=25
10/21/21 17:48:26 ME-IXXXX: [765CD31C7B904D45A5C7341920035017.MAI] DNS resolved to the following record: IP Address=104.47.100.36, Family=2, Type=1, Protocol=6
10/21/21 17:48:47 ME-E0038: [765CD31C7B904D45A5C7341920035017.MAI] Communications Error: Socket connection to (ourdomain)-com.mail.protection.outlook.com failed (error 10060). The host was either not contactable or it rejected your connection. Socket Family = 2; Host=104.47.100.36; Port=25
10/21/21 17:48:47 ME-E0059: [765CD31C7B904D45A5C7341920035017.MAI] Message Delivery Failure. Attempt (0): Could not connect to mail server for domain ((ourdomain).com). The remote mail server could not be contacted at this time. Message has been requeued.
10/21/21 17:58:04 ME-I0026: [765CD31C7B904D45A5C7341920035017.MAI] Sending message
10/21/21 17:58:04 ME-IXXXX: [765CD31C7B904D45A5C7341920035017.MAI] DNS resolved to the following record: IP Address=104.47.101.36, Family=2, Type=1, Protocol=6
10/21/21 17:58:25 ME-E0038: [765CD31C7B904D45A5C7341920035017.MAI] Communications Error: Socket connection to (ourdomain)-com.mail.protection.outlook.com failed (error 10060). The host was either not contactable or it rejected your connection. Socket Family = 2; Host=104.47.101.36; Port=25
10/21/21 17:58:25 ME-IXXXX: [765CD31C7B904D45A5C7341920035017.MAI] DNS resolved to the following record: IP Address=104.47.100.36, Family=2, Type=1, Protocol=6
10/21/21 17:58:46 ME-E0038: [765CD31C7B904D45A5C7341920035017.MAI] Communications Error: Socket connection to (ourdomain)-com.mail.protection.outlook.com failed (error 10060). The host was either not contactable or it rejected your connection. Socket Family = 2; Host=104.47.100.36; Port=25
10/21/21 17:58:46 ME-E0059: [765CD31C7B904D45A5C7341920035017.MAI] Message Delivery Failure. Attempt (1): Could not connect to mail server for domain ((ourdomain).com). The remote mail server could not be contacted at this time. Message has been requeued.
It seems like (our domain)-com.mail.protection.outlook.com is blocking the service.