A common challenge when migrating mailboxes to any Microsoft Exchange platform, where the end user devices use Outlook 2007/2010 with caching, is the initial generation of OST files.

Outlook with cached mode is the most efficient method in terms of network footprint – estimated to be approx 2.5k per user. However, you have to complete the caching of the local mail file copy first (the OST file).

This post examines one particular mitigation against the burst of network traffic resultant from the initial OST file generation.

Network saturation can occur if any one of the following scenarios are realised:
* Mailbox sizes are large
* The Exchange Mailbox server resides across the WAN
* WAN link is not very fast, and is overloaded

This may result in loss of network service to the users for an unknown period of time, whilst the initial OST files are generated.

Microsoft have written many detailed tech notes on this subject, with a popular mitigation for large mailboxes being to pre-seed the OST file back at the Data Centre. To do this, you disable cached mode on the end user device (via GPO maybe), then when the mailbox is on the new Exchange system a device in the Data Centre location is used to start Outlook as the user in cached mode. Once the OST file is generated, it is shipped down to the remote site where the end-user’s device is.

Next, copy the OST file into the correct path in the user’s Window Profile (check the Microsoft information for this). Then modify the user’s Outlook MAPI profile Advanced settings to set the OST file location. For this step it may appear as though the OST file is already selected, but actually it isn’t. You must browse to the OST file (that was copied earlier), and re-select it with the OK button. Save the changes to the Outlook MAPI Profile. Then go back into the Outlook MAPI Profile, and enable cached mode.

Next, start Outlook, and monitor the folder in the Windows Profile, which contains the OST file, to ensure that it is this file that is being updated, and that no new OST file has been generated.

The Microsoft steps for this task are very detailed, but do not focus on the need to re-select the OST file in the Outlook MAPI Profile, once it has been deployed on the end-user device.