Using an Imap server with OE6 client over an internet connection.
Recently some new messages, many but not all from one sender, seem to
disappear. That is, I have OE set to download only headers. I see a
message header but as soon as I click on it, the header shows a line
through it and instead of downloading the message, I get an error
along the lines of "message no longer available on server." If I go
into the same mailbox via webmail, all the messages are there and
viewable.

The mailbox limit on the server is around 85 MB and shows about 78 MB
used. Also, some messages still come through ok. No settings on the
client have changed in a long time.