Our Setup

External email arrives at a Postfix server, is scanned, and then forwarded via settings in transport (using the RELAY:[{ip-address}] for a given address) to an Exchange 2007 server. Some users are on Exchange, but a few are still on the Postfix server (they will be moved in the near future).

IMAPS is provided for external connections via Dovecot; in-house, IMAP is provided for the Gateway and native MAPI is used for Exchange/Outlook. Blackberries are connected via BIS, which uses Dovecot as a reverse-proxy IMAPS service to connect to Exchange (when the mailbox exists on Exchange, otherwise it connects to the mailbox on the gateway).

The Issue

We have a user that, when they forward an email on their Outlook client, they get a duplicate of the original message on their Blackberry. When I say duplicate, I mean that they have a copy of the forwarded version of the message (i.e. their version of the message that they obtained hitting the forward button), and a copy of the original message that shows up at the same time.

The expected behavior is to just see the forwarded message, not the forwarded message and a 2nd copy of the original message.

We've only seen this with Outlook users that also have a Blackberry. Other IMAP clients, such as OS X Mail or Thunderbird, do not exhibit this behavior when connecting to the Exchange server; forwarded messages work as expected.

The Questions

• what is causing this to happen?
• why does it only affect Outlook/Blackberry setups, and not TBird/Blackberry or OSX-Mail/Blackberry?
• how do we get it to stop, before people go insane and never forward messages again?