I agree with Snake as regards setting this up as a scheduled task.
However, it is not necessary, and probably not preferable, to run it via
an AutoExec macro. Put it in a separate macro. In the Command Line of
your scheduler, you can specify the macro, somewhat like this...
"C:\PathToAccess\Msaccess.exe" "C:\PathToDB\YourDB.mdb" /x YourMacro
However, if your default email client is Outlook, you may have a problem
with a security warning, that can't be controlled from Access. If this
turns out to be a problem, some people have reported success with a
third-party utility
http://www.contextmagic.com/express-clickyes/
Otherwise, you may have to look at another approach, e.g. Office
Automation, rather than the SnedObject action. There is some good
information at
http://www.granite.ab.ca/access/email.htm