You can check for the @ and the . character in each mail address
Sub test()
' J.E suggestion "?*@?*.?*" instead of *@*
For Each mycell In Columns("B").Cells.SpecialCells(xlCellTypeConstants)
If mycell.Value Like "?*@?*.?*" Then
'do nothing
Else
MsgBox "Wrong E-mail address in " & mycell.Address
End If
Next
End Sub