We had a question last week about some messages sent to a local mailing list not reaching the members of the list. When we looked at the logs on the list server we saw that the messages were being discarded as duplicates/loops. This is an explanation of why this happens and how to avoid it.
Every mail message has identifying label associated with it which should be globally unique. This label is called a message-id (commonly shorted to msgid). The system we use to run our mailing lists (Sympa) relies on this to stop looping messages being sent to a list repeatedly. In the version we use at the moment the list of msgids that have been seen is only cleared out when the server is restarted for maintenance – this happens irregularly (later versions expire entries in the cache after a fixed time). This is a reasonably common technique to protect lists for mail-loops – I remember implementing it in the locally written MLM when I worked at Mailbase.
The system deliberately sidelines the message silently because it thinks this is a possible loop and sending a message to the sender has a fair chance of making things worse.
Unfortunately some mail programs will create messages with identical msgids. I believe that some versions of Outlook do this if you use the “Resend” option on an existing message. The workaround is to not use “Resend” unless you’re resending a message that failed to deliver. Some old versions of the Pine mail program generated duplicates occasionally because they used the current time to create the msgid but missed out one of the components(hours, minutes or seconds – can’t remember which).
We’ve found another instance in which Outlook will send messages with identical msgids and that’s using templates. If you use Outlook templates in non-cached mode (more specifically if you use a template created when in non-cached mode) then messages created from that template will all have the same msgid. See discussion at http://social.technet.microsoft.com/Forums/en-US/exchangesvrtransport/thread/890412c8-f992-4973-b504-f1d069b0266f/
The suggested workaround for this is to change to using Outlook in Cached mode (see http://www.ncl.ac.uk/itservice/email/staff-pgr/outlook/cachedexchangemode/) and then recreate the templates (you need to create new templates because the fault is attached to the template). If for some reason cached mode isn’t suitable all the time (for example if you regularly use different desktop machines) you just need to turn it on when creating the template.