OLEXP has an option when you create an email to send it at a later date. Create a new email. It's in different places depending on whether you're using 98, 2000 or XP - in 98 & 2000, its under Options on the toolbar, in XP it's File->Send Later.
If you really want to pick up mail later, you can work offline & only connect when you want to send/receive - you can also set your connection to hang up after checking email, but if you do that, disable automatic checking and uncheck the send immediately option. I personally haven't done this since the old compuserve newsgroup days (when the connection charged by the minute). Then it was connect, get the latest headers, disconnect, mark the ones I want to read, connect & DL those, disconnect & reply at my leisure, connect to send & get all new headers, disconnect ...