Continue working even after the browser is closed

I have a page "Page1" with some details and which I click on Submit button, it will start a collection say "lstUserDetails" in a loop and send mails. 
Say , i have 5 items in the collection (lstUserDetails)
If i am closing the browser after click on Submit button, I will receive only 1 or 2 mail as the 2 items only processed before browser unload.
My requirement even if the browser is closed, all 5 mails should be send.
the "lstUserDetails" is stored in a Session in "Page1"