In RFC 2822 (defining E-Mail) is defined, that no line SHOULD be longer than 78 chars (excluding CRLF) and MUST not longer than 998 characters. With quoted-printable longer lines will be broken into more lines, ending each with a '=' until the real linebreak is reached. Conforms a mail to the standard, if it contains lines longer than 78 (or 998) characters but is encoded with quoted-printable?
There are arguments, that this isn't compliant, because the receiving mail-client has longer lines after decoding the quoted-printable message.
EDIT: To clarify the question in the way as asked by David Cary: Yes, I mean the quote-printable encoded mail should be compatible to quoted-printable, means the lines are no longer than 76 characters. But the decoded messages may have longer lines than this limit. So my question is: Is client software implementing RFC 1521 supposed to handle indefinitely long lines after decoding quoted-printable text content? This is answered yes with both answers so far (thanks) with the restriction that it is discouraged by Netiquette (RFC 1855). But Netiquette even limits a line length to 65 characters, a limit nearly nobody adheres to.
Best Answer
I'm not sure what you are asking:
a receiving mail client finds long lines before decoding quoted-printable
Say the quoted-printable encoding software on the transmitting end simply quoted non-printable letters, making the resulting encoded line longer than the original line, without ever adding "soft line breaks", resulting in an encoded line longer than the limit.
This is non-compliant.
-- Wikipedia: quoted-printable, paraphrasing RFC2045 Page 21.
the encoded lines are short, but a receiving mail client finds long lines after decoding quoted-printable
That is compliant with RFC2822 and RFC2045, and should be supported by all software.
However, creating such messages is discouraged by several Netiquette Guidelines, including Page 3 of RFC 1855 "Netiquette Guidelines".