A mail composing program claiming compliance with this specification MUST ensure that any string of non-white-space printable ASCII characters within a "*text" or "*ctext" that begins with "=?" and ends with "?=" be a valid encoded-word. ("begins" means: at the start of the field-body or immediately following linear-white-space; "ends" means: at the end of the field-body or immediately preceding linear-white-space.) In addition, any "word" within a "phrase" that begins with "=?" and ends with "?=" must be a valid encoded-word.
A mail reading program claiming compliance with this specification must be able to distinguish encoded-words from "text", "ctext", or "word"s, according to the rules in section 6, anytime they appear in appropriate places in message headers. It must support both the "B" and "Q" encodings for any character set which it supports. The program must be able to display the unencoded text if the character set is "US-ASCII". For the ISO-8859-* character sets, the mail reading program must at least be able to display the characters which are also in the ASCII set.