You can gain a lot of control over searching your mail by using "Raw Queries" passed straight to the Spotlight Engine.

To input a "Raw Query" go to "Advanced Find" - Shift-Command F. And switch the Query Type on the left side of the input box to "Raw Query", it's at the bottom of the list.

From there you can start assembling queries like:

com_microsoft_outlook_author_email_addresses == "*@example.com"c || kMDItemAuthors == "*Paul*" && kMDItemRecipients != "*Roy*"  

Queries accept the boolean operators || for "or" and && for "and." Splat (*) is the wildcard character. By appending a "c" after a quoted search value the search becomes case insensitive.

These are all the fields I'm aware of, but there are probably more:

com_microsoft_outlook_author_email_addresses == someone@example.com  

This field matches on the e-mail address of the author

kMDItemAuthors == "*Richard*"  

This field matches on the name of the author

kMDItemRecipients == "John Smith"c  

This field matches on the name of a recipient

com_microsoft_outlook_accountID  

Numeric account ID

com_microsoft_outlook_attachments  

List of attached files

com_microsoft_outlook_categories  

Numeric categories

com_microsoft_outlook_completed  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_flagged  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_folderID  

Numeric folder ID

com_microsoft_outlook_forwarded  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_has_attachments  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_has_reminder  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_has_text_content  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_hasDueDate  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_hasStartDate  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_isFromMailingList  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_messageSent  

Timestamp in format: 2013-10-24 09:38:23 -0800

com_microsoft_outlook_primaryCategory  

Numeric category?

com_microsoft_outlook_priority  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_recordID  

Numeric record ID

com_microsoft_outlook_redirected  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_repliedTo  

Boolean value? 0 for false, 1 for true

com_microsoft_outlook_size  

Unsure, boolean?

com_microsoft_outlook_unread  

Boolean value? 0 for false, 1 for true

I'll update as I find more.