The web-browswer version of Outlook Email needs to have the distribution list address(es) configured as 'Safe mailing lists'.
To do this, login to Outlook Email, click on the Settings menu (the little gear-wheel symbol near the top-right of the window) and then select Options from the dropdown menu. Here is a partial screenshot.
Now you should be looking at the Options screen. On the left, under Options, select Mail ⇨ Junk email ⇨ Safe mailing lists.
In the box marked 'Enter an address here', type in the distribution list address email@example.com
Click on the plus (+) sign, and then the Save button.
Your screen should then look something like the screenshot below.
Note: If you are a committee member, you would also need to add firstname.lastname@example.org
When you are done, just click on 'Options' to return to your messages.