Hi Kalpana,
There are 2 Components related to sending EMails.
1)CRMCMP_CCS_EML
2)BT126H_MAIL
Once you find the relevant component then goto the Method IF_BSP_WD_TOOLBAR_CALLBACK~GET_BUTTONS corresponding to the view.There you can write your logic and set the property ENABLED of the internal table to SPACE for the button you want to disable.
For Component BT126H_MAIL the View is:BT126H_MAIL/send_screen
For Component CRMCMP_CCS_EML the View is:CRMCMP_CCS_EML/MailBody
Regards.
Deepak Ramchandran.