I just accidentally sent the exact same e-mail twice to the same recipient (my computer was acting sluggish and I double clicked too many times). To avoid this, is there a way to either delay sending e-mails or set sync intervals for the desktop client? I'm using Lotus Notes 8.5.3
You can edit your location document, go to the Mail tab and change "Mail file location" to Local (make sure to have a local replica of your mail). This opens up an additional field for "Transfer outgoing mail if" which you can set to a number. If you set this reasonable high mails will be held in the local mail.box database. You can then open mail.box and delete mails which should not be sent.
Using a local mail replica is something you should consider anyway for performance reasons. With a local replica you do not need to access the server for your mail file operations.
Here's a blog entry that describes the steps: http://blog.darrenduke.net/darren/ddbz.nsf/dx/notes-slow-use-a-local-replica.htm.
Here is IBM documentation on using and administering local mail file replicas in general: http://www.ibm.com/developerworks/lotus/library/local-mail-replicas/