Is there a best time to send emails?
When is the worst time to send emails?
A quick Google search will deliver umpteen results for articles that claim to give a definitive answer to these questions.
The truth is, it depends.
Your industry, audience, geographical reach, the temperament of the recipient, their workload etc., will all have a bearing on the success rate of your email campaign or newsletter.
Not the most useful answer I know, but I stumbled across this infographic on Email on Acid that offers an insight into the best times of day to send your emails. There are of course no guarantees with it, but it does give you an understanding of when a good window of opportunity may appear.