The Search Mail window can be accessed by using the Search button on the main toolbar or by using the menu options Utilities 
Search Mail
This function allows you to quickly and easily find a message within the current user's folders, based on the presence of a word or words.
A search can be restricted to the current folder (with or without sub-folders), or all folders.
A new system has been introduced which radically speeds up the search - if a word is not present somewhere in the message database, the search will tell you this extremely quickly. If it is present, the search will take longer as a check is made for the exact whereabouts of messages containing the word or phrase.
After mail is collected, the search database will be updated in the background. This may explain any unexplained activity, which is nothing to be concerned about.
To: From: and CC: entries in the header will be included as well as the Subject: If you have upgraded to Version 8.02 from an earlier versions, the Search database must be re-built for these items to be indexed on existing messages.
If you delete a lot of mail, then it is a good idea to rebuild (Utilities Rebuild search database) the Search database from time to time, to clear out the items now no longer relevant. This will reduce the size of the Search database and make searches much quicker.
Once messages have been found, you can list them by different fields by clicking on the relevant field column header. A second click will reverse the order of sort by that field (eg from ascending to descending).
Click a HotSpot on the image below for further information:
This allows you to search for patterns rather than just text strings, and allows the search of just header or body, see above.
This is a complex subject - for more detailed information do an internet search for it, or see www.regexbuddy.com/regex.html. Here are some brief details:
foo will only find foo, f* will find such strings as fo, foo, fouks etc
receive[s|d] will find receives and received.