John wrote:

And what about compacting the pst-file? Doesn't that improve the
performance?

Actually no. It inhibits performance. When you remove a message from a
PST, the space it occupied is mared free for reuse and is not returned to
the Windows file system It remains available for Outlook to use again
without having to request additional space from the file system. Allocating
additional disk space can be an "expensive" process. By keeping the space
in the PST, Outlook does not have to wait for the file system to respond
with an additional allocation.

Maybe it's a stupid suggestion regarding he is using IMAP. (I don't
know what that is, but I sometimes compact my pst file).

Since with IMAP all messages remain on the server, I don't see why archiving
is of any use unless the PST acting as the IMAP server cache is getting
close to the 2GB limit (and I wouldn't venture past 1.5GB). If the PST is
getting close to the limit, then the only thing to do is to permanently
remove messages from the server. Archiving can do that.
--
Brian Tillman