With the recent Rollup 8 for Service Pack 1, the Exchange team previewed a change designed to reduce the impact of inbound SMTP messages on the named property map for a store. Jason talked about it here. The gist of the change is that before, random e-mails with random x-headers could exhaust the map, making it impossible for legitimate applications, like Outlook, to map new named properties. With Rollup 8, those x-headers will only be promoted if they’ve already been mapped, or if the mail was submitted by an authenticated user.
Jason’s latest post details where we’re taking this in SP2 and Exchange 2010. An x-header will only be promoted to a named property if the mapping already exists, either because it was mapped before SP2 was applied, or a MAPI based application requested the mapping. We know this breaks a few scenarios, but felt it was a reasonable tradeoff to preserve the stability of the Exchange server.
Jason has asked for feedback on this change, so please head over there and give it. Thanks!