SmtpException Unable to read data from the transport connection net io connectionclosed

Question

I am using the SmtpClient library to send emails using the following   SmtpClient client   new SmtpClient    client Host    hostname   client Port   465  client DeliveryMethod   SmtpDeliveryMethod Network  client UseDefaultCredentials   false  client EnableSsl   true  client Credentials   new NetworkCredential  User    Pass   client Send  from hostname    to hostname    Subject    Body      The code works fine in my test environment  but when I use production SMTP servers  the code fails with an SmtpException  Failure sending mail   with an inner IOException  Unable to read data from the transport connection  net io connectionclosed    I ve confirmed that firewalls are not an issue  The port opens just fine between the client and the server  I m not sure what else could throw this error

User · Accepted Answer

EDIT  Super Redux Version  Try port 587 instead of 465  Port 465 is technically deprecated     After a bunch of packet sniffing I figured it out  First  here s the short answer   The  NET SmtpClient only supports encryption via STARTTLS  If the EnableSsl flag is set  the server must respond to EHLO with a STARTTLS  otherwise it will throw an exception  See the MSDN documentation for more details   Second  a quick SMTP history lesson for those who stumble upon this problem in the future   Back in the day  when services wanted to also offer encryption they were assigned a different port number  and on that port number they immediately initiated an SSL connection  As time went on they realized it was silly to waste two port numbers for one service and they devised a way for services to allow plaintext and encryption on the same port using STARTTLS  Communication would start using plaintext  then use the STARTTLS command to upgrade to an encrypted connection  STARTTLS became the standard for SMTP encryption  Unfortunately  as it always happens when a new standard is implemented  there is a hodgepodge of compatibility with all the clients and servers out there   In my case  my user was trying to connect the software to a server that was forcing an immediate SSL connection  which is the legacy method that is not supported by Microsoft in  NET

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For anyone who stumbles across this post looking for a solution and you ve set up SMTP sendgrid via Azure     The username is not the username you set up when you ve created the sendgrid object in azure   To find your username     Click on your sendgrid object in azure and click manage   You will be redirected to the SendGrid site  Confirm your email and then copy down the username displayed there   it s an automatically generated username    Add the username from SendGrid into your SMTP settings in the web config file    Hope this helps

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In my case the web server IP was blocked on the mail server  it needs to be unblocked by your hosting company and make it whitelisted  Also  use port port 587

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Change port from 465 to 587 and it will work

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Try this   Here is the code which i m using to send emails to multiple user     public string gmail send                 using  MailMessage mailMessage           new MailMessage new MailAddress toemail       new MailAddress toemail                          mailMessage Body   body              mailMessage Subject   subject              try                               SmtpClient SmtpServer   new SmtpClient                    SmtpServer Credentials                       new System Net NetworkCredential email  password                   SmtpServer Port   587                  SmtpServer Host    smtp gmail com                   SmtpServer EnableSsl   true                  mail   new MailMessage                    String   addr   toemail Split          toemail is a string which contains many email address separated by comma                 mail From   new MailAddress email                   Byte i                  for  i   0  i  lt  addr Length  i                        mail To Add addr i                    mail Subject   subject                  mail Body   body                  mail IsBodyHtml   true                  mail DeliveryNotificationOptions                       DeliveryNotificationOptions OnFailure                       mail ReplyTo   new MailAddress toemail                   mail ReplyToList Add toemail                   SmtpServer Send mail                   return  Mail Sent                             catch  Exception ex                                string exp   ex ToString                    return  Mail Not Sent     and ther error is     exp

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Change your port number to 587 from 465

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To elevate what jocull mentioned in a comment  I was doing everything mention in this thread and striking out    because mine was in a loop to be run over and over  after the first time through the loop  it would sometimes fail   Always worked the first time through the loop   To be clear  the loop includes the creation of SmtpClient  and then doing  Send with the right data   The SmtpClient was created inside a try catch block  to catch errors and to be sure the object got destroyed before the bottom of the loop   In my case  the solution was to make sure that SmtpClient was disposed after each time in the loop  either via using   statement or by doing a manual dispose    Even if the SmtpClient object is being implicitly destroyed in the loop   NET appears to be leaving stuff lying around to conflict with the next attempt

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If you are using an SMTP server on the same box and your SMTP is bound to an IP address instead of  Any Assigned  it may fail because it is trying to use an IP address  like 127 0 0 1  that SMTP is not currently working on

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I ve tried all the answers above but still get this error with Office 365 account  The code seems to work fine with Google account and smtp gmail com when allowing less secure apps    Any other suggestions that I could try    Here is the code that I m using  int port   587  string host    smtp office365 com   string username    smtp out mail com   string password    password   string mailFrom    noreply mail com   string mailTo    to mail com   string mailTitle    Testtitle   string mailMessage    Testmessage    using  SmtpClient client   new SmtpClient          MailAddress from   new MailAddress mailFrom       MailMessage message   new MailMessage               From   from            message To Add mailTo       message Subject   mailTitle      message Body   mailMessage      message IsBodyHtml   true      client DeliveryMethod   SmtpDeliveryMethod Network      client UseDefaultCredentials   false      client Host   host      client Port   port      client EnableSsl   true      client Credentials   new NetworkCredential               UserName   username          Password   password             client Send message       UPDATE AND HOW I SOLVED IT   Solved problem by changing Smtp Client to Mailkit  The System Net Mail Smtp Client is now not recommended to use by Microsoft because of security issues and you should instead be using MailKit  Using Mailkit gave me clearer error messages that I could understand finding the root cause of the problem  license issue   You can get Mailkit by downloading it as a Nuget Package    Read documentation about Smtp Client for more information  https   docs microsoft com es-es dotnet api system net mail smtpclient redirectedfrom MSDN amp view netframework-4 7 2  Here is how I implemented SmtpClient with MailKit          int port   587          string host    smtp office365 com           string username    smtp out mail com           string password    password           string mailFrom    noreply mail com           string mailTo    mailto mail com           string mailTitle    Testtitle           string mailMessage    Testmessage            var message   new MimeMessage            message From Add new MailboxAddress mailFrom            message To Add new MailboxAddress mailTo            message Subject   mailTitle          message Body   new TextPart  plain     Text   mailMessage             using  var client   new SmtpClient                          client Connect host   port  SecureSocketOptions StartTls               client Authenticate username  password                client Send message               client Disconnect true

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In case if all above solutions don t work for you then try to update following file to your server  by publish i mean  and a build before that would be helpful    bin- gt  projectname dll    After updating you will see this error  as i have solved with this solution

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In my case  the customer forgot to add new IP address in their SMTP settings  Open IIS 6 0 in the server which sets up the smtp  right click Smtp virtual server  choose Properties   Access tab  click Connections  add IP address of the new server  Then click Relay  also add IP address of the new server  This solved my issue

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Does your SMTP library supports encrypted connection   The mail server might be expecting secure TLS connection and hence closing the connection in absence of a TLS handshake

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My original problem is about intermittent sending failures  E g  First Send   succeeds  2nd Send   fails  3rd Send   succeeds  Initially I thought I wasn t disposing properly  So I resorted to using    Anyways  later I added the UseDefaultCredentials   false  and the Send   finally became stable  Not sure why though

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This error is very generic  It can be due to many reason such as The mail server is incorrect  Some hosting company uses mail domainname format  If you just use domain name it will not work  check credentials host name username password if needed Check with hosting company    lt smtp from  info india uu com  gt           lt  -- Uncomment to specify SMTP settings -- gt           lt network host  domain com  port  25  password  Jin   userName  info india xx com   gt         lt  smtp gt       lt  mailSettings gt

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I ran into this when using smtp office365 com  using port 587 with SSL  I was able to log in to the account using portal office com and I could confirm the account had a license  But when I fired the code to send emails  I kept getting the net io connectionclosed error    Took me some time to figure it out  but the Exchange admin found the culprit  We re using O365 but the Exchange server was in a hybrid environment  Although the account we were trying to use was synced to Azure AD and had a valid O365 license  for some reason the mailbox was still residing on the hybrid Exchange server - not Exchange online  After the exchange admin used the  Move-Mailbox  command to move the mailbox from the hybrid exchange server to O365 we could use the code to send emails using o365

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Prepare  1  HostA is SMTP virtual server with default port 25 2  HostB is a workstation on which I send mail with SmtpClient    and simulate unstable network I use clumsy  Case 1 Given  If HostB is 2008R2 When   I send email  Then   This issue occurs   Case 2 Given  If HostB is 2012 or higher version When  I send email  Then The mail was sent out   Conclusion  This root cause is related with Windows Server 2008R2

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For outlook use following setting that is not giving error to me  SMTP server name smtp-mail outlook com  SMTP port 587

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If your mail server is Gmail  smtp google com   you will get this error when you hit the message limit  Gmail allows sending over SMTP up to only 2000 messages per 24 hours

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You may also have to change the  less secure apps  setting on your Gmail account   EnableSsl  use port 587 and enable  less secure apps    If you google the less secure apps part there are google help pages that will link you right to the page for your account   That was my problem but everything is working now thanks to all the answers above

User · Answer

removing   client UseDefaultCredentials   false     seemed to solve it for me

[c#] SmtpException: Unable to read data from the transport connection: net_io_connectionclosed

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