Delivery failure to Exchange public folder

I set up a new public folder in our Exchange server to receive emails from external SMTP server. The intent was to store emails from customers for management perusal. As the folder was going to contain sensitive business information, I allowed access to it to management group only. Unfortunately, the delivery to the folder failed - the email was returned to sender with no explanation. Exchange server's event log showed nothing, but when I increased Exchange logging level, the following cryptic message showed up in the log:

The following call : EcLocallyDeliverMsg2 to the store failed. Error code : 1238 (Message-ID <046e01c83c04$e24bbfd0$5101a8c0@csltd.intranet> will be NDR'd). MDB : 87b7cb4d-7e6b-47fa-be39-85b0d7995226. FID : . MID : . File : C:\Program Files\Exchsrvr\Mailroot\vsi 1\Queue\NTFS_e1be85a401c83c0400000024.EML.

Well, that didn't clarify much. A quick internet search turned up this Microsoft KB article: http://support.microsoft.com/kb/873393, but that definitely was not my case.

Giving the issue more thought, I realized that in attempt to secure the access to the folder as much as possible I went a little too far by disallowing any access to Anonymous users. Now, when an external SMTP server submits an email to Exchange server, this is anonymous access as far as Exchange server is concerned. Thus the attempt to submit a message to the public folder is denied! The solution was to allow "Contributor" access to the Anonymous user.

The bottom line is that if an Exchange public folder is to receive email from an external SMTP server, Anonymous users should be allowed "Contributor" access.