Why does MAPI seem to discard/favour 'Return-Path's in PR_TRANSPORT_MESSAGE_HEADERS

Hi,

When we save a mail using MAPI, it seems to show different behaviors in handling thePR_TRANSPORT_MESSAGE_HEADERS , depending upon what 'Return-Path's it contains

1. Two return paths are present, one seems to be correctly formatted:

<property id="PR_TRANSPORT_MESSAGE_HEADERS" type="String">
Return-Path: <info@odmailout.officedepot.com>
Return-Path: info@odmailout.officedepot.com
...</property>

On saving the above using MAPI, only the first 'Return-Path' is retained.

2. Two 'Return-Path's are present and both are correctly formatted (in angled brackets)

<property id="PR_TRANSPORT_MESSAGE_HEADERS" type="String">
Return-Path: <phpbot@mysite4now.com>
Return-Path: <onlinesecurity@hsbc.co.uk>
...</property>

Questions:

A). Why is this different logic for handling return paths by MAPI?
B). How does MAPI handle more than one return-path in the transport header?

Thanks
Sandy