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.