The RFC defines that any kind of boundaries or separators must terminate with a CRLF (\r\n) and not only LF (\n).
The mimemail.inc file uses only LF and therefore the messages are broken on some clients, like MS Outlook for example. Actually messages sent using mimemail doesn't work at all over Outlook (HTML body is ignored and the text version comes truncated).
The proposed patch fix this issue.
Frederico Caldeira Knabben
"Support Open Source Software"