Tapping on the magnifying glass at the top of the message list, reveals the search toolbar:
By starting to type in the search toolbar, message will automatically appear if they match the search query.
Unread: Displays unread message
Conversations: Display message in conversation style
Attachments: Display messages with attachments.
All the tools in the Search toolbar can be used in combination.
For example: To search for all unread messages with attachments that contain the word airmail will look like this: