The inventor of World Wide Web - Tim Berners-Lee has mentioned some good rules for e-mailing on his World Wide Web Consortium page:
What not to email
Email is safe unless it contains programs. (Data and documents are fine, programs are not). If you send me a program, I will not run it, as it could damage my system and could be a virus.
Note: Documents for Microsoft word, Excel, and possibly other Office programs tend to execute programs (scripts) in what you would expect to be harmless documents. These can expose my machine to viruses, because these programs do not (it seems) prevent scripts from running within a document when it received by email. Please do not send me Microsoft Office documents.
If you are sending text, please send it as plain text or HTML. If you use your favorite word process, slide tool, etc, and send it in that program's format, then you are forcing me install proprietary software on whatever machine I read them on. .
If your email is sent from Microsoft Outlook, and contains an attachment, I will be more likely to discard it as I understand that a famous series of viruses in 2001 resulted from Outlook's tendency to execute scripts in email, and used up a huge amount of my and my colleague's time.
What you can email
These are all good document standards: Plain text messages, HTML (sometimes called rich text) pages without scripts, Photos (JPEG files, PNG, GIF), SMIL, RDF/XML N3 and so on. All these can be sent as messages or as attachments to messages. I can read them with a variety of software programs, and they cannot contain viruses, unless there is a serious bug in the code I use to read them. If you don't need anything else, then use plain text.
These are good rules when emailing anyone.
Tim Berners-Lee's Blog
BBC: Web inventor fears for the future
