I quote Ayman in https://bugzilla.mozilla.org/show_bug.cgi?id=920546#c15 here:

>> A single match results in replacing the "manually" entered recipient with
>> the correct contact. What should the app do if there are >1 matches?
>
> Take the first result in the list of contacts returned, ni? me if in your opinion this behaviour produces undesirable results or you see issues.

So that means we want to take the first one if we have more than one exact match.