Thanks Ken
I tried below xml
customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui"
onLoad="OnLoad"
ribbon
tabs
tab idMso="TabReadMessage"
group id="ClearV2"
label="ClearV2"

button id="Button2"
size="large"
label="ClearV2"
screentip="ClearV2 tip"
onAction="OnButton2"
getEnabled="CallBackIsEnable"
imageMso="HappyFace"/
/group
/tab
/tabs
/ribbon
/customUI

I didn't get any error ,but my button was not displayed on Ribbon for
existing mail.What went wrong?
I'm using single vb file & 2 xml files -one for compose & other for
read.The vb file has below 2 functions
Public Function GetCustomUI(ByVal ribbonID As String) As String
Implements Office.IRibbonExtensibility.GetCustomUI
If ribbonID = "Microsoft.Outlook.Mail.Compose" Then
If gblnLicense = False Then Return Nothing
Return GetResourceText
("Clearv_Outlook_Add_In.rbnComposeMail.xml")
ElseIf ribbonID = "Microsoft.Outlook.Mail.Read" Then
Return GetResourceText
("Clearv_Outlook_Add_In.rbnReadMail.xml")
End If

End Function

Protected Overrides Function RequestService(ByVal serviceGuid As Guid)
As Object
If serviceGuid = GetType(Office.IRibbonExtensibility).GUID
Then
If ribbon Is Nothing Then
ribbon = New rbnComposeMail()
End If
Return ribbon
End If

Return MyBase.RequestService(serviceGuid)
End Function

Thanks again