Named properties are a legacy mechanism by which Exchange reserves a property ID in a limited addressable space for use by applications. The history of and issues with named properties are discussed in detail at the MS Exchange Team Blog here but the important note is that, with the explosion of Internet e-mail and numerous applications requiring Exchange to allocate named properties for debatably useful reasons, the number of named properties in Exchange can grow quickly toward predefined quota thresholds. At this point users could be prevented from sending e-mail from Outlook.
Exchange 2003 and Exchange 2007 databases can support a maximum of 32,767 named properties. This number is not configurable and is limited by the size of the data type. However, by default, Exchange 2003 and Exchange 2007 have quota thresholds at 8,192 and 16,384 named properties, respectively. As such, many of my clients that deployed Outlook 2007 against an Exchange 2003 back-end experienced issues with named properties quota thresholds due to the introduction of additional named properties.
Upon reaching your named properties threshold, Exchange will record a number of event log errors (generally MSExchangeIS event ID 9667) and e-mail sent from Outlook clients could be bounced back with a 5.2.1 NDR. E-mail sent from OWA was unaffected in the client environments I’ve worked. Unfortunately, I have seen similar issues when deploying Exchange 2010 into an Exchange 2003 environment, with or without Outlook 2007/2010, where Exchange 2010 users would see e-mail sent to Exchange 2003 users bounced back. E-mail sent between Exchange 2010 users only or between Exchange 2003 users only were unaffected.
To resolve this issue, you need to increase the quota thresholds associated with named properties via a registry change per mailbox database in Exchange 2003 (or Exchange 2007) and then dismount/mount each database. This process is described at http://support.microsoft.com/default.aspx?scid=kb;EN-US;820379 and I strongly recommend that you consider proactively increasing Exchange 2003 named properties quotas to 16,384 before moving any mailboxes to Exchange 2010 to avoid any potential issues. In addition, there is a great MS Exchange Team blog post here that discusses named properties and some misconceptions in detail.
For more in my series on Exchange 2010 Notes from the Field, please click here.