Claws Mail is the best open source email client. Only one important feature that Sylpheed offers is missing.

Navigating through results of a recursive search in "Quick Search" that
are spread over multiple subfolders is only possible by manually
searching the magnifying glass icons on subfolders (see also bug 3207).
This procedure is cumbersome on a 800+ subfolder tree containing 100.000+ emails.

Mairix is not sufficient since it only links emails. It is neither possible to
delete/move/flag the original emails from the mairix search results folder
nor to navigate to the original emails. Additionally, mairix does not
support searching for flagged messages (F:f) and unread messages (F:-s)
in Claws Mail (MH folder format).

Ideally, recursive search results would be displayed like in Sylpheed in a separate persistent virtual folder. Failing that, recursive search results could be displayed in a separate window. The third best option would be displaying only folders with matching emails and hiding non-matching folders in the folder tree.

Thanks,
Inka