Comment 11 for bug 438116

We switched to direct delivery as our local SMTP server was fast enough, and it avoided a problem we saw with queued. We were creating emails that where undeliverable, such as a bad To: address - a real world SMTP server has many more checks than the simple ones in Launchpad and the Zope infrastructure. The SMTP server refused to attempt delivery, and they stayed in the queue indefinitely until someone manually cleared themout. The dud messages were retried each and every time the delivery thread kicked in - the queued delivery was not smart enough to remove emails with permanent failures.