The document provides tips for avoiding common mistakes when writing informal emails. It discusses choosing an appropriate tone for different recipients, using "reply all" judiciously, keeping messages concise, double checking attachments, not sending emails in anger or without proofreading, using clear subject lines, and not over-relying on email as a communication method. Ten specific mistakes are outlined, such as using the wrong tone, hitting "reply all" excessively, writing messages that are too long, forgetting to attach documents, emailing the wrong person, being too emotional, not using the "delay send" function, having vague subject lines, not reviewing emails for errors, and sending unnecessary emails.