Hallo Dave,
This is again related to the same header problem that's being experienced by some in gmail, and qmail-scanner.
All mail headers should only be terminated by a \n, not a \r\n.
Header fields are lines composed of a field name, followed by a colon (":"), followed by a field body, and terminated by CRLF.
[ http://www.faqs.org/rfcs/rfc2822 ]
Note: If messages are not received, try using a LF (\n) only. Some poor quality Unix mail transfer agents replace LF by CRLF automatically (which leads to doubling CR if CRLF is used). This should be a last resort, as it does not comply with RFC 2822.
[ http://www.php.net/manual/en/function.mail.php ]
qmail is very strict, a patch exists to accept \n:
"qmail by default violates the 'be liberal in what you accept' rule when receiving mail via smtp. It enforces strict \r\n behaviour... which is exactly what the standard dictates."
[ http://www.arctic.org/~dean/patches/qmail-0.95-liberal-lf.patch ]
Doesn't seem to be a roundcube specific problem - the used MTA is to blame...
Best regards, Lars