How to add a signature to all outgoing SMS messages in Google Voice

For the non coders not wanting to use the Python answer, the solution is shortcuts.

1. Select "Show: Unread" instead of the default "Show: All" in the top bar towards the right.
2. Try * (asterisk) then A (selects all)
3. Shift I (marks all selected as read, then goes to next page)

I marked over 400 messages as read in under a minute.

If you happen to use AutoHotKey, here is a script that will perform the above by typing shift-F9. Just be sure to first click on the "unread" option at the top.

F9::
   SendInput *a!
   Sleep, 500
   SendInput I
   Sleep, 500
   SendInput n
return

You may want to investigate IF This Then That and maybe their recipe that allows you to automatically forward a e-mail message to a phone via SMS. This recipe may not fully work for you, depending on your needs, but this is a start.

ifttt is truly a godsend when it comes to connecting different services.