rejected email -- smtp lines too long

Getting this error/rejection notice after sending some emails:

"550 5.6.0 Lines longer than SMTP allows encountered; message rejected"

Some emails get through to the same user(s) that have these errors, some do not. Saw there was a similar issue in version 6, but saw no reference to an issue/fix/resurrection in 7. Spaces and Line Breaks are removed from CSS definitions I do have the option to include site style sheets selected.