I think this worked in previous versions.

In the composer, you start typing a name into the To: field, and it offers you some possible completions. I was entering the host name for the organisation that I knew the email address belonged to, because I couldn't remember the contact person at that organisation.

E.g. Joe Smith <eat@joes.com> - in this example I was typing "joes" and hoping for it to match the email address. But it didn't, it gave me one wrong completion, so it's not an issue with generating too many possibilities. It simply didn't match for some reason.

Perhaps it doesn't handle partial matches, but requires them to be typed from the start of the address; "eat" in this example. If that's the case, then as a bare minimum, it should also handle matches starting from after the @. I would expect that forgetting the contact person at an organisation would be a fairly common reason for needing autocomplete.