[raxxorrax]

On public websites I would agree, but all our mail clients have auto-completion. So would we need to turn that off? Would probably disable half the company.

I don't know who Ashley Madison is but that sounds far beyond sensible protection. Given, auto-completion is restricted to employees plus some locally saved contacts. It is just the standard outlook-exchange setup.

[unionpivo]

You can set up what (groups) autocomplete as a admin on both outlook and g suite (probably other providers).

Otherwise it's only people you have been in contact with.