MS tech support has sent this:

Symptom

Emails sent from Outlook Live accounts to external listserv show up with attachment missing. 

Cause

When the Outlook Live user is sending out to the listserv contact, it’s sending the message out in RTF(Rich Text Format), which the listserv, or something else along the way, doesn’t like, and therefore the message is getting munged(i.e. messed up). 

Resolution

Run the following powershell to turn on TNEF for any e-mail sent out to the Internet:

Set-RemoteDomain * -TNEFEnabled $false

OR

Run the following command to disabled RTF for a contact:

Set-MailContact <listserv contact> -UseMapiRichTextFormat never

For all contacts: Get-MailContact | Set-MailContact -UseMapiRichTextFormat never 

http://technet.microsoft.com/EN-US/library/4738bf25-39b8-4433-bd64-1d60252c2832(EXCHG.150).aspx
http://help.outlook.com/en-us/140/gg263346.aspx


We already have a "RemoteDomain" setting for the UGA Listserver (to prevent OoOs), and checking, it appears 
   -TNEFEnabled 

is neither $true nor $false (no default setting).  So I will guess that one of the recent MS updates changed the way Office365 (we no longer have an on-prem Exchange server) sends mail.  "Language= Exchange" would accept it; this will prevent it from being sent.

I tend to think when we have moved to R16, the "Misc-Options= KEEP_EXCHANGE_DATA" would be better, but this will solve the immediate problem for our 1.8d box.

Thanks to everyone for their help on this.
--Joe

________________________________________
From: LISTSERV Site Administrators' Forum <[log in to unmask]> on behalf of Ben Parker <[log in to unmask]>
Sent: Thursday, May 29, 2014 1:27 AM
To: [log in to unmask]
Subject: Re: Attachments, 1.8d and Outlook?

On Wed, 28 May 2014 08:05:48 -0700, Shinn Wu <[log in to unmask]> wrote:

>Misc-Options= KEEP_EXCHANGE_DATA

This is for 15.0 and later.  For 1.8d the equivalent is

Language= Exchange

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