I keep getting sync errors when trying to send an email to a group in my Outlook address book

Hi kraz219

Thank you for posting to Microsoft Q&A.

Based on your description, I understand that you are experiencing issues when sending emails to a mailing list (contact group) in Outlook for Mac, where you receive the error message “At least one recipient is not valid. Participant has empty SMTP address” followed by subsequent sync errors.

The issue occurs because Outlook for Mac uses local (client-side) Contact Groups, which store all members’ information locally rather than on the Exchange or Microsoft 365 server. If any member in the group has an invalid or missing email address (for example, a contact entry without an SMTP value), Outlook will not be able to process the message, and the send action will fail for the entire group. You can find detailed information here:

Error expanding Contact Group in Outlook Desktop - Microsoft Support

I can't send or receive messages with Outlook for Mac - Microsoft Support

I'd like to share some recommended solutions that you can try:

Option 1: Recreate the Contact Group

• Open the existing Contact Group and copy all members to a text editor or Excel > Review each address and remove entries without a valid SMTP format (e.g., missing “@domain.com”).
• Delete the old Contact Group and create a new one and re-add the cleaned list of members and test by sending to a small subset first.

This will clear any corrupted or invalid entries that may be causing the sync error.

Option 2: Use a Server-based Distribution List (Recommended)

You can consider creating a distribution group on the server (If you don't have admin access, ask your IT administrator). This method is more reliable and doesn't rely on local Outlook sync.

You can refer to: Create a group in the admin center - Microsoft 365 admin | Microsoft Learn

I hope the information I provided is helpful. If you have any questions, please feel free to ask.

If the answer is helpful, click "Accept Answer" and kindly upvote. If you have additional questions about this answer, click "Comment". 

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.