Many email servers are configured to automatically generate a plain text version of a message and send it along with the HTML version, to ensure that it can be read even by text-only [[email client]]s, using the <code>[[MIME content type|Content-Type]]: [[MIME#Alternativealternative|multipart/alternative]]</code>, as specified in RFC 1521.<ref>[http://tools.ietf.org/html/rfc1521#section-7.2.3 RFC 1521 7.2.3. The Multipart/alternative subtype]</ref><ref>{{cite web|url=http://www.codestone.ltd.uk/software/docs/csmail/tn1010-11-2.pdf |title=TN1010-11-2: Multipart/Alternative – Gracefully handling HTML-phobic email clients. |accessdate=2012-06-24}}</ref><ref>{{cite web|url=http://www.wilsonweb.com/wmt5/html-email-multi.htm |title=Sending HTML and Plain Text E-Mail Simultaneously |publisher=Wilsonweb.com |date=2000-04-28 |accessdate=2012-06-24}}</ref> The message itself is of type <code>multipart/alternative</code>, and contains two parts, the first of type <code>text/plain</code>, which is read by text-only clients, and the second with <code>text/html</code>, which is read by HTML-capable clients. The plain text version may be missing important formatting information, however. (For example, a mathematical equation may lose a superscript and take on an entirely new meaning.)
Many{{Citation needed|date=September 2009}} [[Electronic mailing list|mailing list]]s deliberately block HTML email, either stripping out the HTML part to just leave the plain text part or rejecting the entire message.{{Citation needed|date=September 2009}}
