Unable to send emails- some domains

Hi Mac77123,

here are some steps you can take to resolve this problem:

1. Confirm the Recipient’s Email Address:

   • Double-check the recipient’s email address for any typos or unnecessary spaces. Sometimes a small error in the address can lead to this issue.
   • If you find any mistakes, correct them and resend the message.

2. Remove Invalid Entries from Auto-Complete List:

   • Invalid entries in your Auto-Complete list (also known as the nickname cache) can cause issues. These entries might be outdated or associated with a different internal identifier.
   • In Outlook, you can remove invalid recipients from the Auto-Complete list:
     • Start typing the recipient’s name or email address in the To field.
     • When the recipient appears in the drop-down list, select them and press the Delete key to remove the cached entry.
   • If you’re using Outlook on the web (formerly Outlook Web App):
     • Remove a single recipient: Click New mail, start typing the recipient’s name, select them, and click the X on the right-hand side.
     • Remove all recipients: Clear your Auto-Complete list by using the light version of Outlook on the web. You can do this by opening your mailbox in an older web browser that only supports the light version or configuring your settings to use the light version.

3. Check the Service Routing Domain:

   • Ensure that the domain has replicated to all Microsoft Exchange Online servers. This process can take up to an hour.
   • Verify that the appropriate MX (Mail Exchange) record exists or has replicated to all applicable Domain Name System (DNS) servers worldwide. This can take up to 72 hours.

For more detailed instructions, refer to this article:
NDR-Fehler 550 5.1.1 bis 5.1.20 in Exchange Online behoben - Exchange | Microsoft Learn