High availability means that the email system is almost always working for you. The higher the availability, the more email will be there for you.
Disaster recovery for email means getting email working very quickly after a, er, disaster. For example, when email is hit by:
Microsoft's been working work to increase Exchange availability over the last few years. Notes has had strong availability features for many years--it deserves to have received more credit for this. For an example of disaster recovery for Exchange, see Cemaphore's MailShadow. ... David Ferris