It's an Asian viburnum, perhaps Viburnum plicatum f. tomentosum 'Shasta'.

Or Viburnum plicatum f. tomentosum 'Mariesii'. I don't know how you tell them apart. But the first part of the name is pretty certain.

Thank you both!

Goodness... how do you tell them apart, Shasta and Mariesii?!

Really the only difference is in size - 'Mariesii' is a larger growing plant than 'Shasta'. Both, if left unpruned, are strongly horizontal in habit, producing branches in layers and in maturity will be quite a bit wider than tall. The flowers are also held on erect peduncles that elevate the flowers above the branches producing an effect when in bloom like white icing on a cake :-) And in pairs along the stems, hence the common name of doublefile.

And there are other cultivars as well.

These are both plants that are best sited where they can be left to grow to size unpruned. The very horizontal layered form is altered if the plant is pruned often (other than to remove a stray vertically growing shoot) and you lose the proper effect (also flowering potential if pruned at the wrong time). You might want to consider relocating to a more suitable location.

IMO, either cultivar is one of the most stunning and showy of spring flowering shrubs!