In a support discussion titled “When do Edits in an old message trigger an update to users / Message Order,” users explore how message edits affect thread sorting and notifications. BCHK observes that editing older messages sometimes moves them to the top of the list and asks whether such edits trigger emails to thread participants. gerhard clarifies that only editing the last post in a topic brings it to the top, while other edits do not. lisajill adds that tagging posts also unintentionally pushes them to the top. ky_metro notes that deleting a necro-post can leave an older post at the top temporarily. BCHK follows up, asking whether either editing the last post or other edits trigger notifications to participants. codinghorror confirms that only entirely new posts generate notifications — updates or edits never do. The consensus is that edits affect message ordering only if they involve the last post, and no edits trigger email notifications to participants.
Sometimes when I edit older messages the message jumps to the top of the message list, other times this doesn’t happen (I believe). Also - I was wondering if these edits ever result in an email to the users who have participated in that thread?
Any help in understanding how this works is greatly appreciated.
If I’m not mistaken, editing the last post in the topic will bring it to the top of the list. All other edits won’t.
Tagging posts has the unfortunate effect of sending them to the top as well. 
Also a deleted necro-post will leave the previous ancient post at the top for awhile.
But will either scenario - editing last post, or other edit - result in that message being resent to the people who have participated in that thread?
Or - are only entirely “new” posts broadcast to the thread participants?
Only new posts will generate notifications. Updates never will.