At the top of the thread list is a search bar. Type the name of a sender, a word from the subject, or a term from the message body. The inbox filters immediately.
The search query is combined with your active folder (Inbox / Sent / Archive / etc.) and any active mailbox or client filter. So you can search only your sales mailbox for the word "invoice", for example.
For global search across the whole Senly portal (clients, tasks, mail, pages) use ⌘K / Ctrl+K.