Mailkitを使用してSSL+POP before SMTPで添付ファイル付き電子メールを送信する (C#)

        public static void Send(string sendto, string sendfrom, string subject, string body,

            string[] attachFileNames,

            string smtpServerHostName, int smtpPort,

            string smtpUserId, string smtpUserPassword,

            string popServerHostName, int popPort,

            string popUserId, string popUserPassword)

        {

            if (isDisable(sendto)) return;

            if (isDisable(sendfrom)) return;

            if (isDisable(smtpServerHostName)) return;

            if (smtpPort == 0) return;

            bool useSmtpAuth = !isDisable(smtpUserId) && !isDisable(smtpUserPassword);

            bool usePopBeforeSmpt = !isDisable(popServerHostName) && popPort != 0

                && !isDisable(popUserId) && !isDisable(popUserPassword);

            var message = new MimeMessage();

            message.From.Add(new MailboxAddress(sendfrom));

            message.To.Add(new MailboxAddress(sendto));

            message.Subject = subject;

            if (attachFileNames.Length == 0)

            {

                message.Body = new TextPart("plain") { Text = body };

            }

            else

            {

                var multi = new Multipart();

                multi.Add(new TextPart("plain") { Text = body });

                foreach (var filename in attachFileNames)

                {

                    var attachment = new MimePart()

                    {

                        ContentObject = new ContentObject(File.OpenRead(filename), ContentEncoding.Default),

                        ContentDisposition = new ContentDisposition(ContentDisposition.Attachment),

                        ContentTransferEncoding = ContentEncoding.Base64,

                        FileName = Path.GetFileName(filename)

                    };

                    multi.Add(attachment);

                }

                message.Body = multi;

            }

            if (usePopBeforeSmpt)

            {

                using (var client = new Pop3Client())

                {

                    client.ServerCertificateValidationCallback = (s, c, h, e) => true;

                    client.Connect(popServerHostName, popPort, true);

                    client.AuthenticationMechanisms.Remove("XOAUTH2");

                    client.Authenticate(popUserId, popUserPassword);

                    client.Disconnect(true);

                }

            }

            using (var client = new SmtpClient())

            {

                client.ServerCertificateValidationCallback = (s, c, h, e) => true;

                client.Connect(smtpServerHostName, smtpPort, true);

                client.AuthenticationMechanisms.Remove("XOAUTH2");

                client.Authenticate(smtpUserId, smtpUserPassword);

                client.Send(message);

                client.Disconnect(true);

            }

        }