sending letters during few hours

sending letters during few hours

Hello!
I noticed that the emails with invoices and "payment thank you" sent to the customer for a very long time - the delay can be 3-4 hours.
I know when the letters come because put checkbox "send me a copy."
Tell me, please, what it can be connected with? may be the reason is the difference in hours or another, or what the another reason?
thanks in advance