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Searching
There are several methods for searching your emails.
One of them is relatively standard, and can be found in the "Edit" menu, it's the "Search folder..." item. This will open a window where you can specify one or more fields to search in: From, To, Subject, and Body. After having specified your criteria, click on the "Back" or "Forward" buttons to navigate through the matching emails, or use "Find all" to select all the matching emails at once. Be aware that searching for text in the body of emails is much slower than searching in its headers, because the body of emails isn't cached by Claws Mail. "Extended Search" mode can be turned on by clicking the relevant checkbox, thus allowing you to use match expressions like those used in "Filtering" rules.
If you're looking at a large email and want to find a particular part of it, you can use the "Find in current message..." item of the "Edit" menu. This works like search in a text document.
The final way of searching for emails is using "Quick Search", which you can display or hide by clicking the little magnifying glass icon under the Message List. It is also accessible from the "Quick Search" item of the "Edit" menu. "Quick Search" is more powerful than the normal search as it can search in standard headers (From, To, Subject) or in "Extended" mode using just about any criteria you can think of. When in "Extended" mode, the "Information" button is visible, enabling you to see the search syntax. An "Edit" button is also available which allows you to quickly create a rule. You can also configure "Quick Search" to search recursively through subfolders, whether it should reset itself when you change folders, and whether to use Type-ahead search (this is a search which results update dynamically if you pause in the typing).
When you hit Enter after having specified the search string, the Message List will shrink to present you with only the matching messages. If you set the search to be recursive, any subfolder of the current one that has matching emails will change its icon to a magnifying glass icon. This way, you can search in your whole mailbox at once. If the search is in Sticky mode, the filter will stay applied when you go to another folder. This can be disturbing at first, as you can forget about it, but is useful in some cases, for example if you want to search in the body of emails and are not sure of which folder contains the searched email: a recursive search on the body of emails in a whole mailbox can be really slow.