As reported in bug #576184 comment #8:

Using 3.22.3 on Ubuntu 16.10 AMD64; I have all messages sync'd locally.  I'm searching for the term "optic", with scope "Message contains".  The search is finding this word in some emails, but not in others.  Here is my best guess at what is happening.

Evolution appears to be searching for partial text on the subject.  It is finding subjects like this: "ThinOPTICS Always With You Lifetime Guarantee Activated".  This email has no instance of "optic" in the body, but it does in the recipient email address.

It is NOT finding several emails with subject "New reply: Unreliable Search" and with this phrase in the body: "the search should be text:optics."  However, if I change the search term to "optics", then these emails ARE included in the search results, and when I select one of them, the text "optics" is highlighted.

I'm concluding from this, perhaps erroneously, that searching for partial text is not being done on the body, but only full word search.

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I was able to reproduce with a Gmail server, but not with a Dovecot server.