Outlook is the commonest email package found in offices and it really is a useful package if you can get to grips with its entirety. There's not a part of it I don't use. Because it comes in sections, it makes teaching a doddle - you can concentrate on just email or skim through each bit as your class prefers. Don't forget that it's very handy to teach things like housekeeping principles and how the server delivers your mail, too.
Sending, receiving, forwarding
CC, BCC
Managing attachments
Creating folders
Display options
Signatures
Filtering
Setting the out-of-office assistant
Mail format
Voting
Tracking and delivery options
Message flags, importance and sensitivity
Saving emails
Creating a contact
Emailing a contact
Creating a distribution list
Creating appointments
Creating meetings, inviting attendees, cancelling or rescheduling
Creating events
Recurring items
Reminders
Marking time and labelling
Allowing others access to your Calendar
Creating, editing and recording progress
Assigning tasks to others
Creating an appointment from a task
Creating, using and changing options
Basic housekeeping principles
Using views
Customising the fields
Archiving