When you send email, you do not necessarily know how the recipients will receive it. They could have a completely different kind of computer to you, or they could be checking their email on a mobile phone, through digital television, and so on.
Sending email in plain text means that it can be read by the recipient no matter what system that are using. It also means that your email might be less likely to be rejected as unsolicited email ('spam') because it will contain no programming language code.
