Exchange Mail Enabled Public Folder NDR Woes

So I spent a ridiculous amount of time today trying to figure out why external users were receiving NDRs when attempting to send mail to a newly created mail-enabled public folder.  I remembered to make sure the related distribution group and the folder itself had "Require that all senders are authenticated" UNchecked in the Message Delivery Restrictions.  What I didn't know was that the public folder permissions (as in from Outlook) had to have Anonymous allowed to create items.  Here's hoping someone finds this tidbit faster than I did by searching today!