I am also yet to encounter anyone who sent me an email between noon on Friday and noon on Sunday who received an undeliverable response. It suggests that they still reside on a TalkTalk server somewhere. Perhaps the place to start looking, TalkTalk, is in the location of the emails that did turn up on Monday (sent after noon on Sunday).
I think it's safe to say that your/our/everyone's mails of similar timescale have now gone for good. Certainly, last time a Pipex outage of this kind happened that was the case. And requests/questions about the missing mails asked on here were just pretty much ignored. You will simply have to like it or lump it. Hopefully though, once all migration is complete, 'Pipex' issues will no longer exist, although it will remian to be seen.