After checking through the sendmail.c code for version 3 and 4, I noticed that the code for multiple recipients and CC which was in version 3 is not included in version 4 in the SendText() function.
This might explain why Mail() does not send to multiple recipients or CC in version 4.0.

This code from version 3 is missing in version 4.
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	tempMailTo = estrdup(mailTo);
	
	// Send mail to all rcpt's
	token = strtok(tempMailTo, ",");
	while(token != NULL)
	{
		sprintf(GLOBAL(Buffer), "RCPT TO:<%s>\r\n", token);
		if ((res = Post(GLOBAL(Buffer))) != SUCCESS)
			return (res);
		if ((res = Ack()) != SUCCESS)
			return (res);
		token = strtok(NULL, ",");
	}

	// Send mail to all Cc rcpt's
	efree(tempMailTo);
	if (pos1 = strstr(headers, "Cc:")) {
		pos2 = strstr(pos1, "\r\n");
		tempMailTo = estrndup(pos1, pos2-pos1);

		token = strtok(tempMailTo, ",");
		while(token != NULL)
		{
			sprintf(GLOBAL(Buffer), "RCPT TO:<%s>\r\n", token);
			if ((res = Post(GLOBAL(Buffer))) != SUCCESS)
				return (res);
			if ((res = Ack()) != SUCCESS)
				return (res);
			token = strtok(NULL, ",");
		}
		efree(tempMailTo);
	}
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