one@example.com, two@example.com, three@example.com

one@example.com
two@example.com
three@example.com

if (cell contains ", ")
get string from ", " to ", " and paste somehow
else "b1"

%s/,/\n/g

Cells(1, 1).Select
Dim curr As String
Dim cnt As Integer
cnt = 0
For i = 1 To ActiveCell.SpecialCells(xlLastCell).Row
    curr = Cells(i, 1).Value
    If InStr(curr, "@") Then
        If InStr(curr, ",") Then
            Dim tmp() As String
            tmp = Split(curr, ",")
            For j = LBound(tmp) To UBound(tmp)
                cnt = cnt + 1
                Cells(cnt, 2).Value = tmp(j)
            Next
        Else
            cnt = cnt + 1
            Cells(cnt, 2).Value = curr
        End If
    End If
Next i