If you're replying to an email event, FogBugz will use the "From" address from the email event you're replying to (unless you're trying to reply to an email that came from FogBugz, but this is a different situation). I.e., FogBugz will do what "Reply" does in most other email applications: reply to the person who sent the message you're trying to replyto.
The correspondent field is not meant to override the "From" address on incoming emails. If you're just clicking "Reply" at the top/bottom of the case page (i.e., not clicking "Reply" for a specific email event), FogBugz will use the "From" address for the last incoming email event.
The correspondent field is used in the following situations:
If you've created a new case, set the correspondent field, and then later click on the "Reply" action, we will use the correspondent field as the default "To" address.
For a case that is created via an incoming email message, FogBugz will automatically set the correspondent field based on the "From" address from the incoming email so it can be associated with other cases sent in from that same address.
The correspondent field is primarily used to group cases that are related to the same customer. I.e., if you're looking at case X whose correspondent is set to "firstname.lastname@example.org", FogBugz will show you "Also sent: Y, Z" if cases Y and Z also have their correspondent set to "email@example.com". You can also search for all of the cases whose correspondent are set to "firstname.lastname@example.org" using the filter bar or a custom search like "correspondent:email@example.com".