Email is just as serious as other kinds of communication so take heed when using it, saying "It was just an email" is not a good approach.
Don't send spam, chain email or dubious mails with viral warnings. Most people do not appreciate getting these kinds of emails and all it does is increasing the amount of emails being sent out.
Very few of the viral warnings and security warnings that are sent out are legitimate, be vary of running attachments and doing supposedly harmless tasks these kinds of emails describe.
If you're writing something that very many people may be interested in you might consider posting it on an appropriate web forum or newsgroup.
It's considered a bad thing to send emails to very many people concurrently, and the more recipients you have, the less likely it is that all the recipients are interested in what you are writing.
Before you send emails with attachments you should consider the following points:
It might be a bad decision to send out emails with large attachments to many recipients at the same time. That's because each attachment take up space both on the email server while being sent, and also on the email recipients accounts.
Not everyone is on a fast connection so sending funny video files to them may not be the best of ideas.
So, instead, host your file on a web server somewhere(like on the web space on your stud account) and send them a link to the file. If the file is placed in a folder that has a webpage in it already(an index.html) no one will be able to freely browse to it.
Do not share sensitive information this way.
If you're sending an attachment it's important to check if the recipient has the required program to view the file. Also, check if he has the same version of the program as you.
Many people send Microsoft office documents without thinking of that some people(especially in university settings) may not have the means to easily open them.
It's therefore a good policy to send either pure text or, if you have pictures or other graphical elements as PDF.
Keep you email folders neat and tidy. Sort the mail, remove spam and remember that attachments in sent mails and received mails usually are duplicated if you've chosen to save them in a more appropriate folder. Your storage space is not infinite.
Email is meant to be a pure text-based mode of communications, and your efforts to make the email pretty may not work as intended in other email programs.
Many people discard fancy emails without reading them, discarding them as spam. Use text and descriptive subjects to avoid this.
Because of this it's a good policy to send emails as pure text, not as html with fancy fonts and colours.
Be wary of attachments you receive, especially from unknown persons or if you've not asked for the file attached. If you use Outlook or Outlook Express there is a greater risk of beeing infected by virals.
Remember that conveying emotions, like saying things in a joking manner, sarcasm et al. is difficult in written form. People may misinterpret you.
Many write emails in a way that makes it difficult for the recipient to understand it. Remove unnecessary sentences and quote below the text.