A system and method for allowing users to associate various e-mails within at least one of the user's mailboxes under a common discussion topic or “thread” for organizational purposes. This user-controlled organization may be carried out regardless if the e-mails which are desired to be associated are from different senders and/or have different subject headings. An e-mail message may be associated to/designated under a user-defined discussion topic or associated to another e-mail. At the user's discretion, information about the e-mail's associated discussion topic and/or associated e-mails may be stored and kept locally with the user's mailbox, or alternatively, information about the e-mail's associations may be transmitted when that e-mail is sent to a recipient user.