Just wanted to let you know about an issue I ran across. Basically, I tried using the Mailchimp integration, but I didn't end up keeping it. So I turned it off in the system settings. But today, I noticed that no blogs were being emailed using the default system mail queue. I even tried using the manual process of adding a blog to the mail queue, but it didn't work. (It said it did, but no emails went into the queue.)
So I went back to that mailchimp integration area and removed all data from the settings and turned all remaining options off. I tried to manually resend notifications and it worked now.
So it looks like having data in one of those fields in the integration section for mailchimp was messing things up, even though the integration was turned off.
Hope this helps!