Let me clarify again, so I'll be sure you understand me

I'm writting Bulletin Board as a CGI, this CGI is the client, and an
IMAP server will be the server.

If I have thousands of messages, and hundreds of threads packing these
messages, I can't show all these threads at once to the user, I have to
split into different web pages.

This CGI, the client, will ask "a003 THREAD REFERENCES UTF-8 ALL" each
time in order to get messages packed as threads.
>From there, this client is able to pick only the first ten threads for
example.

But if I have thousands of messages, the IMAP answer can be quite long,
right?
That's my main scalability concern.

Also, I can refine with a date instead of ALL, but then I may have
incomplete threads, am I right?

Again, thanks!
Greg