Outlook express stores e-mail messages on the local computer, so if someone has one e-mail account configured on more than one machine, e-mails will be stored in the machine in which they were read (downloaded) and will not be accessible from other machines, although all e-mail messages are related to the same e-mail account.
I am not 100% certain of Outlook Express’s abilities, but if it is similar to other versions of outlook, you can customize it to leave a copy on the server. This option will allow ANY computer you access to view the same set of e-mails.
Hope this helps!
He may also have different e-mail accounts set up on the different computers. Email clients can usially handle several different accounts at the same time. His Home computer amy be configured to work with home related accounts, his work system may be set up to access work related accounts.