Using New Outlook on Windows for personal email, calendar, and contact management
There was a fix posted on Page 5 of this forum post that corrected this issue for me. None of our Microsoft email users could send to any Yahoo addresses from our business domain. Yahoo updated their security settings recently, and they are requiring two forms of DNS security verification for Microsoft email accounts to be able to send to Yahoo accounts. Yahoo only required one until very recently, and Microsoft 365 email accounts only provide one form of DNS record verification when sending emails by default. I added 2 DKIM records to our Office 365 email domain today, and it did take about 8 hours to work itself out, but all users can now send to Yahoo freely. Adding two proper DKIM CNAME records to your DNS record will fix the issue. This link gives you the instructions for adding the necessary DKIM records to your domain, just make sure you use the proper syntax: https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/email-authentication-dkim-configure?view=o365-worldwide