With more and more people using smart phones for email it may be a good time to think about the time of day you send out your mailings.
The reason I bring this up is because email is changing. It used to be a case of booting up the pc to check your mail. Now for a lot of users its "beep beep" not so good if it happens to be 3am.
How are you dealing with this? Set times or staggered due to time zone differences. I think this is something that should be addressed before it becomes an issue.