Is quoted-printable enough to make a mail compliant with the line-length-restriction posed in RFC 2822?
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.
Lines of quoted-printable encoded data must not be longer than 76 characters. To satisfy this requirement without altering the encoded text, soft line breaks may be added ... These soft line breaks also allow encoding text without line breaks (or containing very long lines) for an environment where line size is limited, such as the "1000 characters per line" limit of some SMTP software, as allowed by RFC 2821.
-- 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".
It's definitely compliant. The whole point of Quoted-Printable, and the rest of the MIME series of RFCs (RFC 2045 through RFC 2049), is to allow the encoding of data that otherwise would not be valid in e-mail. RFC 2822 explicitly (and repeatedly!) points readers at those RFCs for information on how to do this.
If you really want to know how complicate it is to built a compliant email composer and parser, then you must watch this video on Youtube: http://www.youtube.com/watch?v=JENdgiAPD6c
Ricardo Signes gives inside views on different RFCs and what stupidity they bring into the real life.
It is 40 minutes long and only scratches the surface of bad and good email "content". After watching you will change your opinion about the email software you thought that it conforms to email standards.