How do I set up gmail to send out an email automatically at a certain time?

You could create a Google Calendar event and set a reminder to go out beforehand.

It’s a workaround, not a great solution. The email will be from “Google Calendar <calendar-notification@google.com>” and the subject will start with “Reminder:”

There are third-party hacks you can use to do this, such as: http://www.lettermelater.com/forum.php?id=2

But no, no pure-Gmail method exists.

How about for Yahoo mail or hotmail or some other email service (I don’t mind getting another email account).

Desktop email clients, like Thunderbird and Outlook, can send & receive Gmail and Yahoo Mail. And they can schedule a send later.

Using Gmail with Thunderbird and Mozilla Suite
http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite

Send Later 1.2.0.0 add-on for Thunderbird
https://addons.mozilla.org/af/thunderbird/addon/4791/

Outlook Inbox boot camp Day 6: Schedule email delivery to co-workers
http://blogs.technet.com/b/hub/archive/2009/10/12/inbox-boot-camp-day-6-schedule-email-delivery-to-co-workers.aspx

@jaytkay GA, but needs a warning. As far as I am aware (and please correct me if I’m wrong) in order for those desktop clients to send an email at a specific time, they need to be open at that time. In other words, you can’t create a message for sending tomorrow at 12pm, then turn off your computer and leave it off all day tomorrow. The message won’t get sent until you turn your computer back on and start Outlook/Thunderbird.

@MrItty Yes, correct, thank you for the clarification. The email program must be running.