Hi Peter,
From the information you post, I understand that when sending emails to your Office 365 accounts, there is an error message “503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.
”. If I have misunderstood anything, please feel free to let me know.
To help resolve this issue, could you provide the following information for us to troubleshoot?
1.Which subscription of Office 365 are you using ?
2.Can you receive emails in OWA(Outlook Web App)? Did you encounter the same error message in OWA?
3.Is there any other people who also encountered the same issue in your company?
If you can send and receive emails successfully in OWA. Please refer to the steps below to create a new profile in Outlook to see if the issue persists:
1.Open start -> control Panel.
2.Click view by Large icons.
3.Click Mail(32- bit).
4.Click to Show Profiles.
5.Click Add button and type the new profile name, click OK.
6. Cancel the following account setting, choose always use this profile, and choose the newly created profile in the dropdown list
7.Click OK.
You can refer to the links below to get more information about creating profiles and add accounts in Outlook:
How to create and configure an email profile in Outlook 2010, Outlook 2007, and Outlook 2003
http://support.microsoft.com/default.aspx?scid=kb;en-us;829918&Product=out2003
Set Up E-Mail in Outlook 2010:
http://help.outlook.com/en-US/140/dd253202.aspx
Set Up E-Mail in Outlook 2007:
http://help.outlook.com/en-US/140/Cc511396.aspx
Given the current situation, you could provide your domain name for us to further research. Since this is your privacy, I will send a private message to you to request your domain name. Therefore, you could reply me through private message.
To access the private message :
1.Please go to the Your details section on the right side of the community site.
2.Click Private messages.
3.Click the subject title of the response to read the message and reply with your domain information.
Thanks,
Linda Wang