Hi @mara2021,
Based on your description of the problem and the message trace offered, A likely cause for this error is your firewall is blocking connections from Microsoft 365 IP addresses. Afterwards, here's a solution you can try to fix the problem: Modify your firewall settings to allow connections from Microsoft 365 IP addresses on TCP port 25 to your on-premises email servers instead of allowing only specific IPs. You can refer to the documentation linked below for a list of the Microsoft 365 IP addresses: Microsoft 365 URLs and IP address ranges - Microsoft 365 Enterprise | Microsoft Learn.
In addition, you also can view delayed mail’s message headers to analyze delay reason:
-View message headers in Outlook on your desktop
- Double-click an email message to open it outside of the Reading Pane.
- Click File > Properties.
- Header information appears in the Internet headers box.
-Copy the headers information into Message Header Analyzer (Message Header Analyzer (mha.azurewebsites.net)) and check the hop’s delay details.
Hope these will be helpful for you!