Hi @pavan kumar ,
Could you send email from on-premises Exchange server to external recipient normally?
1.I noticed that the on-premses Exchange server could not receive the mail sent by O365 internally or externally initially, and then you set abc.com as the FQDN in HCW. So how do you set up HCW to run in your current environment? Please make sure that the FQDN set in your HCW is your on-premises Exchange server.
2.About external url. According to my knowledge, external url of autodiscover only could set up in Exchange 2010, and normally it is empty by default and does not need to be set. Other External urls are usually set to https://fqdn/<service>.
3.Please log in to Microsoft 365 admin center -> Settings -> Domains. Check if you have successfully added your on-premises Domain to Office 365.
4.In addition to Port 25, please make sure that you opened the URL, IP and Port required by Office 365.
Please refer to: Office 365 URLs and IP address ranges
In addition, if possible, please share the complete NDR message with us. Pay attention to covering your personal information.
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