Hi @Joey Yu ,
From your description, please kindly check if set up SPF, DKIM and DMARC for your custom domain in Microsoft 365 completely. Please also make sure if your network bypassed Office 365 URLs and IP address ranges which help to deliver email to yahoo account.
You may also contact your Microsoft 365 admin to find out what happened to an email message by running a message trace in the Exchange admin center. For more information, see Message trace in the modern Exchange admin center.
In addition, I found a similar thread that mentioned the issue may be caused by that Yahoo had identified Microsoft as "abusive”. This is the issue between Yahoo and Microsoft, instead of your issue. All emails I am attempting to send to Yahoo/Aol accounts are bouncing - Microsoft Community
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