By default when you create a distribution group (also known as a distribution list) external email addresses are unable to send emails to the group.
To allow external email addresses to use the group you will need to disable the ‘Require that all senders are authenticated’ option.
Please note, this may open your server and users to unwanted spam. You may want to do this in conjugation with message restrictions to minimise unwanted emails from external email addresses.
How to allow external senders to use a distribution list
- In the Exchange Management Console, expand the ‘Organization Configuration’ node, and then select the ‘Distribution List’ node
- Once you have located the desired Distribution Group, double-click on the object
- Select the ‘Mail Flow Settings’ tab
- Select ‘Message Delivery Restrictions’ then click on the ‘Properties’button
- un-tick the ‘Require that all senders are authenticated’ option
- Click ‘OK’ then ‘OK’ again to save the changes.
- External email addresses are now able to use the distribution list.