Mail (Mac OSX) Search Features

Mac Mail (Email cleint) - Search bar
Doing searches here only searches for mail in your client. The default search is only for one criteria at a time.

When you type in a word in the search bar, a menu appears allowing you to refine your search. The options are obvious:
All Mailboxes, Inbox, Entire Message, From, To, Subject

Advanced search
Using Syntax
keyword1 from:some_user subject:some_word
Example: chromatography from:agilent subject:HPLC

Using Boolean operators (AND, OR, NOT)

Example:
Chemdraw AND Gaussian - This will search for all the mails with both keywords.
Chemdraw OR Gaussian - This will search mails that contains either (or both) of the keywords.
Chemdraw NOT Gaussian - This will search mails for keywords Chemdraw but will exclude the mails that include the keyword Gaussian.

Note: To make these searches more advanced you can combine syntax and boolean operators.