If you have multiple email addresses delivering to a single account, it is often very useful to be able to reply from a specific address. For example, you may have a personal email address and also a generic address such as accounts@ which both go to the same account (using aliases or distribution lists). Normally you probably want to send mail from your personal address, but sometimes you may wish to send a mail from one of the other addresses such as accounts@.
In most mail clients (such as Thunderbird or our webmail), this is simple to set up and the correct address will be picked when replying (so if someone emailed you as email@example.com, the From address when replying would also be set to firstname.lastname@example.org automatically. Unfortunately, Outlook does not handle this very well, but with a little effort it is possible to alter the From address and the procedure is documented in this walkthrough.