When monit receives a system error regarding sockets (e.g. "Resource temporarily unavailable"), this text is pasted in the error description and also the mail message, including the subject.
The error string itself comes from strerror() and contains a newline at the end. monit does not handle this, leading to the subject lin ending with two newlines. This messes up the email message as two newlines declare the end of the header.
There should be a routine that checks that whatever is placed in the subject does not contain newlines.