I sent out technical newsletter, made a serious mistake maybe, would like to follow up and correct it, ideally by replying to my first message - but currently I can only do it as a new section it seems ... which is a bit difficult for the audience to connect. Maybe this can be added as a new enhancement
Until this is implemented, if you are operating on enwiki you can always ask DannyS712 bot to run through and fix the message (already approved bot task).
That being said, for implementation: every mass message that is sent has an HTML comment with the user who sent it and a permalink to the version of the distribution list used. If this were changed to also include the time the message was sent, then that comment could be used to identify the section to post in