On the EmailVision blog, Tim Watson explains why you shouldn't worry too much about the best time to send an email.  

The answer as to why timing is not so sensitive lies in how people engage and process their email inbox.

In a small poll the folks over at EmailVision ran, 94% of people said that when returning to their inbox they scan ALL new or recent unread emails. This means your position in the inbox is relatively unimportant.

Your email will be scanned for a delete or read decision regardless of whether it's the top email or number 25. Your email will live and die by much stronger factors than time of day such as:

• Subject line and from name 
• Content relevance 
• Previous experience of your emails 
• Brand loyalty and engagement

Trying to time an email to a 2 to 5 hour slot is relatively unimportant. Your email will be deleted based on the above factors and not whether it was first seen at 10am or 2pm.

More important than time of day timing, is timing with regard to the customer lifecycle. Right message, right person, right time is often quoted. However, time doesn't mean 9am or 10am.

Perfect timing is reaching the customer when they are actively:

• Identifying a need 
• Researching options to fulfil a need 
• Deciding which offer to choose.

Read more here: Campaign send time isn't important