When creating a custom rule in Menu>Rules you might have tried to use a condition to find specific word in a mail header.

Trying to add a single word will produce the error: Header words must be specified as "Name: Value" pair.

This is because emails have a numerous mail headers which contain specific information - these include From: or Sender: for original sender of the message, Date:, Received: information, etc. If you're creating your rule based on a specific message, you can check the actual headers by right-clicking inside the email > Open Mail Header... and find which header name your word is in.

Then just use both the header name and the word in the format Name: Value in the rule, for example From: John will affect all messages where 'John' is part of the visible name or email address of the sender.