If you use email mail shots as part of your marketing strategy you should consider the time that the email is sent. I have a few (rather large) companies that chose to send emails out at 4am in the morning! This is extremely annoying and has, at times, caused me to unsubscribe to any future mailings. This is not what you want and therefore it is very important to consider this when it comes to the mail out scheduling.
Many systems will give you feed back of what time a mail out is opened and this information can be vital to future marketing. Depending on who your recipient is and what type of product or service you are marketing can have a strong influence on popular opening times, for example, a mail out that is aimed at other businesses rather than an individual will probably best being sent out during normal working hours.