Check and make sure you are not hitting any "limits" on sending on your account. If you are, there could be a delay while they pause the sending (hourly rates, etc.)
That's why I'm doing my own email serving... it's not got a "high" reputation (yet).. but as time goes on and more and more use of it occurs, it will get better. I've done all I can to prevent the "spam" flag being used on them - but I've had stuff go to SPAM from some big sites even lately.
Mine is viewable (that screenshot came) from the Dashboard App on the web for Mandrill.
I don't use mandrill any longer (and haven't for about a year) so the warning in this is normal I believe for no activity