Getting “process cannot access file” error when deleting files after sending email

I am getting error as mentioned below:

The process cannot access file "E:\TempPDFs\Sample.pdf" because it is being used by another process

I happen to send the pdf from email and after email is sent i need to delete the Sample.pdf file. The code that i have written doesn't work

FileInfo DeleteFileInfo = new FileInfo(directoryPath + "\\" + filename + ".pdf");
                            if (DeleteFileInfo.Exists)
                                File.Delete(directoryPath + "\\" + filename + ".pdf");

here directorypath is E:\TempPDFs, filename is Sample

UPDATED:

public static void SendMail(string fromAddress, string[] toAddress, string[] ccAddress, string[] bccAddress, string subject, string messageBody, bool isBodyHtml, ArrayList attachments, string host, string username, string pwd, string port)
    {

        {
            try
            {
                if (isBodyHtml && !htmlTaxExpression.IsMatch(messageBody))
                    isBodyHtml = false;
                // Create the mail message
                MailMessage objMailMsg;
                objMailMsg = new MailMessage();
                if (toAddress != null)
                {
                    foreach (string toAddr in toAddress)
                        objMailMsg.To.Add(new MailAddress(toAddr));
                }
                if (ccAddress != null)
                {
                    foreach (string ccAddr in ccAddress)
                        objMailMsg.CC.Add(new MailAddress(ccAddr));
                }
                if (bccAddress != null)
                {
                    foreach (string bccAddr in bccAddress)
                        objMailMsg.Bcc.Add(new MailAddress(bccAddr));
                }

                if (fromAddress != null && fromAddress.Trim().Length > 0)
                {
                    //if (fromAddress != null && fromName.trim().length > 0)
                    //    objMailMsg.From = new MailAddress(fromAddress, fromName);
                    //else
                    objMailMsg.From = new MailAddress(fromAddress);
                }

                objMailMsg.BodyEncoding = Encoding.UTF8;
                objMailMsg.Subject = subject;
                objMailMsg.Body = messageBody;
                objMailMsg.IsBodyHtml = isBodyHtml;

                if (attachments != null)
                {
                    foreach (string fileName in attachments)
                    {
                        if (fileName.Trim().Length > 0 && File.Exists(fileName))
                            objMailMsg.Attachments.Add(new Attachment(fileName));
                    }
                }

                //prepare to send mail via SMTP transport
                SmtpClient objSMTPClient = new SmtpClient();

                if (objSMTPClient.Credentials != null)
                {

                }
                else
                {
                    objSMTPClient.UseDefaultCredentials = false;
                    NetworkCredential SMTPUserInfo = new NetworkCredential(username, pwd);
                    objSMTPClient.Host = host;
                    objSMTPClient.Port = Int16.Parse(port);
                    //objSMTPClient.UseDefaultCredentials = false;
                    objSMTPClient.Credentials = SMTPUserInfo;
                    //objSMTPClient.EnableSsl = true;
                    //objSMTPClient.DeliveryMethod = SmtpDeliveryMethod.Network;
                }
                //objSMTPClient.Host = stmpservername;
                //objSMTPClient.Credentials
                //System.Net.Configuration.MailSettingsSectionGroup mMailsettings = null;
                //string mailHost = mMailsettings.Smtp.Network.Host;
                try
                {
                    objSMTPClient.Send(objMailMsg);
                }
                catch (SmtpException smtpEx)
                {
                    if (smtpEx.Message.Contains("secure connection"))
                    {
                        objSMTPClient.EnableSsl = true;
                        objSMTPClient.Send(objMailMsg);
                    }
                }
            }
        }
    }

let me know if any query

Thanks!