Sreekanth,
NACE
Select V3 Billing.
Click on Output types
In the next screen select your Z output type and click on "Mail Title and Texts"
Here for EN maintain as "Service_Invoice_&XNAST-OBJKY&.
After this Click on Details ICON
in the Mail tab maintain Program as SAPLV61B and Form Routine as TEXT_SYMBOL_REPLACE.
Now when the output type is triggered,you will get to see the mail getting triggered in SOST with DOCTITLE as "Service_Invoice_0001234567".
You can also refer to SAP Note 537832.
K.Kiran.