My sister has a problem with emails that she has received disappearing from
her Inbox after she has replied to them. I don't think it is a Rule because
I think that happens before the download.

She is still trying to find a pattern but so far it looks like if she
replies to an incoming email, it disappears at some time after that. She is
able to see it again in the Sent folder.

Any clues?

TIA,

Tom