@Data
public class MailSender {
    private String Subject;
    private String Text;
    private String SetTo;
    private String SetFrom;
    private Map<String, String> Attachments;
}

原因： 实体类属性必须遵循驼峰命名

@Data
public class MailSender {
    private String subject;
    private String text;
    private String setTo;
    private String setFrom;
    private Map<String, String> attachments;
}