IM2000 is even suited to off-line readers, too.

M> Think of how you might decide to read a thread on a mailinglist:
M> you read the first message, and then you may decide not to even
M> look at the other messages in the thread.

That's how one decides to read a thread if one has an "always on" 
connection.  The habits of "off-line" users are somewhat different.

M> [...] if my mother has to dial up several times during emailing, [...]

Then she isn't batching her actions up as "off-line" users usually do,
but is rather acting more like an "always on" user.

I mentioned in an earlier message a hypothetical recipient MUA tailored
for off-line usage that allowed one to go through the list of 
notifications, marking them variously for "fetch message", "fetch 
summary", or "refuse", and then to perform all of the requested actions
in one go.  I should point out that this model isn't all _that_ 
hypothetical.  Netscape Collabra (i.e. the Usenet reader bit) first
obtains summary information for messages and then allows one to mark 
a set of messages and then download their actual headers and bodies 
"for off-line reading".  The IM2000 off-line recipient MUA model of 
operation that I've been describing is little more than a trivial 
extension of this.