Find the MAPI sessions : http://www.calazan.com/exchange-2007-mapi-session-exceeded-the-maximum-of-32-objects-of-type-session/

PR_EMAILADDRESS : http://msdn.microsoft.com/en-us/library/cc842372.aspx
Valid values for PR_ADDRTYPE
SMTP and EX
http://www.tech-archive.net/Archive/Development/microsoft.public.win32.programmer.messaging/2009-02/msg00077.html

LEGACY_DN
http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_23125036.html

Possible issue :
http://groups.google.com/group/microsoft.public.exchange.admin/browse_thread/thread/60622a5e5d5d5d1b?pli=1

How does address resolver works ?
http://technet.microsoft.com/en-us/library/bb430743.aspx

In 2k3 Server

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exchange-mapi-connection.c:738: exchange_{mapiutilgetrecipients}() - object has a recipient without a PR_SMTPADDRESS
PR_{NDRREASONCODE}: 1
PR_{SUPPLEMENTARYINFOUNICODE}: <novell-cox.pro.com #5.1.1>
PR_NDRDIAGCODE: 35
PR_REPORTTIME: Thu 21 May 2009 12:13:05 PM IST IST
PR_{REPORTTEXTUNICODE}: The e-mail account does not exist at the organization this message was sent to.
Check the e-mail address, or contact the recipient directly to find out the correct address.
PR_{NDRSTATUSCODE}: 511
PR_{RECIPIENTNUMBER}: 1
PR_{RECIPIENTTYPE}: 1
PR_INTERNETCPID: 1252

NDR Demistified - http://www.computerperformance.co.uk/exchange2003/exchange2003_NDR.htm
Problem in MIxed Mode 5.5 -> 2003 ? - http://technet.microsoft.com/en-us/library/bb124640%28EXCHG.65%29.aspx
Routing Connector : http://forums.msexchange.org/m_1800490750/printable.htm