Using constructor info.magnolia.module.mail.templates.MailAttachment(File, String, String, String) results to an UnknownHostException while java mail is trying to get the file on Windows.
The error comes from method MailAttachment.setPath(String) which constructs a URL with file:// + absolutePath. On *nix platforms it gives us a url like file:///tmp/myfile, but on Windows the url is file://C:/temp/myfile which is wrong (root slash is missing).
I'd fix this issue by changing URL construction to :
[143] this.url = new URL(FILE_URL_PREFIX + "/" + StringUtils.removeStart(aPath, "/"));
I tested this fix on both Debian and Windows platforms.