IT CAN BE DONE....
IN THE FIELD MAPPING , WHERE EVER YOU MAPPED THE EMAIL ADDRESS YOU NEED LITTLE CODING.
DOUBLE CLICK IT. IT WILL SHOW THE SCRIPT EDITOR, HERE CHECK THE ADDRESS IS THERE ARE NOT , IF IT THERE THEN PASS THE '/' NO DATA SYMBOL , IF ADDRESS IS NOT THERE THEN PASS WHAT EVER YOU ARE SENDING..
If you want to overwrite it, then just move the field from the source structure to the other structure as normal in your field mappings. If it already exists and you don't want to overwrite it, you will first check to see if it exists, if so, then you will not pass it from the source structure to the target. You your LSMW add the code as you see here. I am assuming that you are doing this for user ids? Right?
data: smtp_addr type adr6-smtp_addr.
select single adr6~smtp_addr into smtp_addr
inner join adr6
on usr21~addrnumber = adr6~addrnumber
and usr21~persnumber = adr6~persnumber
where usr21~bname = source-user_id.
if sy-subrc = 0 and not smtp_addr is initial.
* then do not fill the target field in the LSMW routine
target-email_address = source-email_address.