Using subsearches to find loosely related events

The number of use cases for subsearches in the real world might be small, but for those situations where they can be applied, subsearches can be a magic bullet. Let's look at an example and then talk about some rules.

Subsearch

Let's start with these events:

2015-02-10 12:59:59 msgid=704783 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=171755 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=668955 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=001404 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=284794 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=362127 from=[e-mail] to=[e-mail]
2015-02-10 12:59:59 msgid=571419 from=[e-mail] to=ronnie@g&r.com

From these events, let's find out who mary has sent messages to. In these events, we see that the from and to values are in different entries. We could use...