Rated 5 out of 5 stars
When internal addresses are used more often than external ones, this addon really simplifies addressing. Wrong recipients are avoided, which adds the advertised security.
My only remark is, that I use two-letter acronyms for recipients, which means I need autocompletion starting with just two letters typed in. Since the addon code defines 5 letters as requirement, I have to manually edit the autocomplete.js and fix that for me. An option defining that limit would be desirable.