Email sending program takes long time

I have to send email from ABAP program to around 45000 employees .

I am using :

-

CALL FUNCTION 'SO_NEW_DOCUMENT_SEND_API1'

EXPORTING

DOCUMENT_DATA = W_DOCUMENT_DATA

DOCUMENT_TYPE = 'HTM'

COMMIT_WORK = 'X'

TABLES

OBJECT_CONTENT = W_OBJECT_CONTENT

RECEIVERS = W_RECEIVER

-

W_RECEIVER is populated with all employee mail ids.

This takes long time to send emails.

Pls guide some methods if available to improve response time.

Thanks in advance.