If you are gone for a period of time, use the automatic reply feature in Outlook advising that you will not be able to respond until you return. Never assume that simply because you have sent a message, it has been read. Take advantage of this feature to ensure a message sent has been read. If an e-mail message is important, reply briefly to the message upon receipt to let the sender know you got it, even if you will send a longer reply later. Try to rid your mailbox of unnecessary attachments, as they are particularly space-consuming.
Plain text is best.
Use mixed case. Upper case letters alone are generally regarded as shouting. Use upper case only for emphasis. Similarly, do not use bold or italics as the mail may end up being read on a system which cannot display them. Always capitalize your sentences and use appropriate punctuation.