Terms you'll need to understand:
Domain controller
Global catalog server
Organization
Active Directory Connector (ADC)
Post Office Protocol version 3 (POP3)
Internet Message Access Protocol version 4 (IMAP4)
Network News Transfer Protocol (NNTP)
DomainPrep
ForestPrep
Active Directory Migration Tool (ADMT)
Simple Mail Transfer Protocol (SMTP)
Techniques you'll need to master:
Upgrading from Exchange Server 5.5 to Exchange Server 2003
Upgrading from Exchange 2000 Server to Exchange Server 2003
Migrating from other messaging systems to Exchange Server 2003
Migrating to Exchange Server 2003 from other Exchange organizations
Configuring Exchange Server 2003 to coexist with other Exchange organizations
Troubleshooting Exchange Server 2003 in coexistence with other Exchange organizations
Configuring and troubleshooting Exchange Server 2003 for existence with other messaging systems
Configuring and troubleshooting Exchange Server 2003 for interoperability with other SMTP messaging systems