Need to change the user names at run time for Output Type MLGR

Hi Friends,

We are using output type MLGR to send a doc 'Report Goods Receipt to Buyer' which is defined

under->Output Types->Mail title and texts.

Its 'Processing Routine' is: RSNASTSO

Form Routine is: SAPOFFICE_AUFRUF

All is well when we use MB02 MLGR, 'Simple Mail', MP, UserName.

I have maintained 3 users in Communication Method in MB02.

It sends the email to SAPConnect.

Now technically I have found the recievers table getting filled and passed to the function module SO_OBJECT_SEND but i am not understanding how do i change the user name at runtime as they are mentioned as numbers in field .
Could any one tell how to identify the user name based on the attached screen shot .

Thanks

Raj