I need to send out ~2000 emails via a sendmail server to email address's from all over the web, should I send them all at once or throttle sending? How well can sendmail handle 2000 emails being sent all at once?
Sendmail will automatically queue outgoing messages. A quick guide to the mail queue: http://www.feep.net/sendmail/tutorial/intro/queue.html
You may also configure the number of child processes that sendmail will use to process the queue, but it shouldn't be necessary.
Hope that helps.
Edit: And a quick guide to viewing the mail queue: http://osr507doc.sco.com/en/MailMsgG/sndmlT18.html