PS HarriJaakkonen :~/Blog/Posts> cat ./420-420-resolveradrambiguous-ambiguous.html 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address 13/11/2015 · 1 min read · Post #28 Exchange Today I got an error I have never seen before. 420 4.2.0 RESOLVER.ADR.Ambiguous; ambiguous address To fix it you need to: Go to Start, Control Panel, Administrative Tools Double-click Services Restart the Microsoft Exchange Transport Service And all is well. Share this: