Despite the almost universal use of email as a communication medium there are at present no accepted standards which govern how a particular email program will display mail.
Depending on which email software you are using you are likely to have the choice of sending emails in one or more of the following formats:
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Plain text
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Rich text
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HTML
Plain text
As the name suggests plain text emails contain no additional formatting such as bold for emphasis. As a general rule anything which can by typed on a standard QWERTY keyboard can be used in a plain text email.
Although formatting in plain text emails is limited, an accessibility convention exists whereby plain text surrounded by * and _ symbols is used to represent emboldened and underlined text respectively.
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*Text in bold*
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_Underlined text_
The big advantage of plain text over other formats from an accessibility point of view is that the recipient is able to use their own personalised settings to either view the message or have it read back by assistive technology.
The disadvantage is that the lack of structure makes navigation very difficult, so plain text messages should be kept as concise as possible.
Rich text
Rich text format enables emails to be visually enhanced with different font styles, colours, bulleted lists etc. Such visual enhancements have the effect of overriding the recipient's own settings and may cause problems for access technology users.
HTML
Emails created in HTML can effectively include anything which may be included in a standard web page. For example, many advertising emails these days include colourful artwork and images created in HTML. Unfortunately, unless produced to accepted standards such as those provided by W3C Web Content Accessibility Guidelines (WCAG), HTML generated emails may not be easily accessible. There are currently no mainstream email applications which produce WCAG 2.0 compliant HTML.
Return to sender?
Because of the lack of accepted standards specifically for email formats you cannot be certain, especially with RTF and HTML formats, that all recipients will be able to access your mail.
How can I make my emails more accessible?
At the present time the only way to be sure of sending out fully accessible email is to use plain text. Indeed, this may well be a preferred option if you regularly communicate via email with a blind or partially sighted person. However, it is wrong just to assume this so where possible always try and find out what your recipient actually prefers!
Email is essentially a system for sending messages, so you could consider sending materials which require extensive formatting as attachments instead. Of course, HTML allows emails to be distributed directly with information conveyed by pictures and graphical elements included. There are web accessibility guidelines for creating HTML based pages and advice can be obtained from the RNIB Access Consultancy - Web Access Centre.
By also offering text only versions of any RTF or HTML based emails you will be able to reach as wide an audience as possible.
More information can be found in Using links and voting buttons and Attachments.