After sending email campaigns for a long time I've tried loads of times and days. There are a few implied rules that seem to make sense but tend not to have any scientific backing.
For example a common thought is that sending an email campaign on a Monday is a bad idea. The thought is that after a weekend in boxes are full and most of its contents if not deemed immediately important are deleted.
Saturday is thought to be a good day if you have a high-powered list. The thought is that they will be checking their emails over the weekend without resorting to their secretary.
I'm about to send an email campaign to my newsletter list at Global Lingo. And I'm planning on Tuesday at around 11am. Seems like a good time to me.
Yesterday I had a thought though as I was walking in the park checking my Blackberry. And it dawned on me that there now is no perfect time. Virtually no one waits until they return to the office to check their emails. Almost every one has a way to check at any time of day or night.
So today I still plan to send my email at 11am, but next time I'll test the time of day and see what the results are. So to finish this off there is no perfect time for all emails just the perfect time for yours. And the only way you'll ever know is to test and test and test.