Maybe the below for just the domain:
Sub test()
Dim rCell As Range
Dim tmp As String
Dim lStr As String, rStr As String
For Each rCell In Range("A2:A" & Range("A" & Rows.Count).End(xlUp).Row).Cells
With rCell
lStr = Left(.Value, InStr(.Value, ".") - 1)
tmp = Left(.Offset(, 1), InStr(.Offset(, 1), ".") - 1)
rStr = Right(tmp, Len(tmp) - InStr(tmp, "@"))
If lStr = rStr Then
.Characters(InStr(.Value, lStr), Len(lStr)).Font.ColorIndex = 3
.Offset(, 1).Characters(InStr(.Offset(, 1), lStr), Len(lStr)).Font.ColorIndex = 3
End If
End With
tmp = vbNullString
Next rCell
End Sub