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Why do I get an error by trying to send a mail with c#?

I'm trying to send an e-mail using C#, but I get this recurring error :
.
Can you explain me what's wrong with my code ?
Here it is :
SmtpClient client = new SmtpClient("smtp.gmail.com");
client.Port = 587;
client.EnableSsl = true;
client.Timeout = 100000;
client.EnableSsl = true;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("myEmailAdress#gmail.com", "myPassword");
MailMessage msg = new MailMessage();
msg.To.Add("receiver#gmail.com");
msg.From = new MailAddress("sender#gmail.com");
msg.Subject = "My subject";
msg.Body = "My body";
client.Send(msg);
MessageBox.Show("Message sent !");

I have encountered same before.
You are getting this error because you haven't set ON on Less secure app access for sender#gmail.com as you are using Gmail SMTP port.
Reason:
Your email has no remotely access permission. You have to configure
it. Suppose you want to sent email from sender#gmail.com so you
have set that permission NO to this account.
How To Set:
You could try like below
Or can open that tab from this link directly Less secure app access
Update:
As per your comment this is for you which has working perfectly since the beginning of my career
public object SendMail(string fromEmail, string toEmail, string mailSubject, string mailBody, string senderName, string senderPass, string attacmmentLocationPath)
{
try
{
MailMessage mail = new MailMessage();
//Must be change before using
SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
mail.From = new MailAddress(fromEmail);
mail.To.Add(toEmail);
mail.Subject = mailSubject;
mail.Body = mailBody;
mail.IsBodyHtml = true;
// mail.Attachments.Add(new Attachment(#attacmmentLocationPath));
SmtpServer.Port = 587;
SmtpServer.Credentials = new System.Net.NetworkCredential(senderName, senderPass);
SmtpServer.EnableSsl = true;
SmtpServer.Send(mail);
return true;
}
catch (Exception ex)
{
return ex;
}
}
Hope that would help.

This code works for gmail, it is very similar to yours with slight differences, but if you try this, and it doesn't work for you, the issue is not the code - perhaps some other network related issue that you are going to need to fix first:
using (var msg = new MailMessage())
{
msg.From = new MailAddress("fromaddress#gmail.com");
msg.To.Add("toaddress#gmail.com");
msg.Subject = subject;
msg.Body = body;
msg.IsBodyHtml = true;
using (var smtp = new SmtpClient())
{
smtp.Host = "smtp.gmail.com";
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.Credentials = new NetworkCredential("xxx#gmail.com","password");
smtp.Send(msg);
}
}

An SMTP client may log in using an authentication mechanism chosen among those supported by the SMTP servers.

cannot send email using office 365 credentials using C#

I am trying to send email with my office 365 credentials using C#. But It is failing to send and getting exception.
How to fix this? Is it possible to send from office 365?
foreach (var filename in files)
{
String userName = "sri#MyOffice365domain.com";
String password = "password";
MailMessage mail = new MailMessage();
SmtpClient SmtpServer = new SmtpClient("smtp.office365.com");
mail.From = new MailAddress("sri#MyOffice365domain.com");
mail.To.Add("senderaddress#senderdomain.com");
mail.Subject = "Fwd: for " + filename;
mail.Body = "mail with attachment";
System.Net.Mail.Attachment attachment;
attachment = new System.Net.Mail.Attachment(filename);
mail.Attachments.Add(attachment);
SmtpServer.Port = 587;
SmtpServer.Credentials = new System.Net.NetworkCredential(userName, password);
SmtpServer.EnableSsl = true;
SmtpServer.Send(mail);
}
Transaction failed. The server response was: 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message.

How many emails are you going to send simultaneously?
foreach (var filename in files)
All Outlook APIs accessed via https://outlook.office.com/api or https://outlook.office365.com/api have a limit is 60 requests per minute, per user (or group), per app ID. So, for now, developers only can make limited APIs calls from the app. Read through the Microsoft Blog Post for more details on REST API call limitations.
Try to use the following code instead:
MailMessage msg = new MailMessage();
msg.To.Add(new MailAddress("someone#somedomain.com", "SomeOne"));
msg.From = new MailAddress("you#yourdomain.com", "You");
msg.Subject = "This is a Test Mail";
msg.Body = "This is a test message using Exchange OnLine";
msg.IsBodyHtml = true;
SmtpClient client = new SmtpClient();
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("your user name", "your password");
client.Port = 587; // You can use Port 25 if 587 is blocked (mine is!)
client.Host = "smtp.office365.com";
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.EnableSsl = true;
try
{
client.Send(msg);
lblText.Text = "Message Sent Succesfully";
}
catch (Exception ex)
{
lblText.Text = ex.ToString();
}

Cannot send Email :( - The remote certificate is invalid according to the validation procedure

public static void SendEmail(StringBuilder sb) {
string mailbody = string.Empty;
MailMessage mail = new MailMessage();
SmtpClient smtpServer = new SmtpClient("aaaaa.com");
smtpServer.Credentials = new System.Net.NetworkCredential("username", "pwd");
//smtpServer.Port = 587;
mail.IsBodyHtml = true;
smtpServer.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
mail.BodyEncoding = System.Text.Encoding.UTF8;
smtpServer.UseDefaultCredentials = true;
smtpServer.EnableSsl = true;
mail.From = new MailAddress("bbbb.com");
mail.To.Add("ccccc.com");
mail.Subject = string.Format("test");
mailbody = sb.ToString();
mail.Body = mailbody;
smtpServer.Send(mail);
//everytime I get this error message and how to handle this, is this a security breach to bypass ssl in a company ? kindly advise the best way to handle this code and to send email.
}

Try the Unexpected “ The remote certificate is invalid according to the validation procedure.”
...tell StmpClient to ignore security with
client.EnableSsl = true;

Error on sending email from C# code

When I send mail to my gmail account, it shows below error.
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Authentication required...
code I am using is below
MailMessage m = new MailMessage();
SmtpClient sc = new SmtpClient();
try
{
m.From = new MailAddress("me#gmail.com");
m.To.Add("me#gmail.com");
m.Subject = "This is a Test Mail";
m.IsBodyHtml = true;
m.Body = "test gmail";
sc.Host = "smtp.gmail.com";
sc.Port = 587;
sc.Credentials = new System.Net.NetworkCredential("me#gmail.com", "passward");
sc.UseDefaultCredentials = true;
sc.EnableSsl = true;
sc.Send(m);
Response.Write("Email Send successfully");
}
catch (Exception ex)
{
Response.Write(ex.Message);
}

Just tried your code, had to fiddle with a couple things but was sent this. Funny because I have done this previously using Gmail smtp (couple years back). But it looks like they are now verifying apps that use their platform.
Either use another smtp server that you are signed up to, or use your own. (there must be a test one that is available online??). Pretty sure sendgrid do a free trial.

using System.Net;
using System.Net.Mail;
string smtpAddress = "smtp.mail.yahoo.com";
int portNumber = 587;
bool enableSSL = true;
string emailFrom = "email#yahoo.com";
string password = "abcdefg";
string emailTo = "someone#domain.com";
string subject = "Hello";
string body = "Hello, I'm just writing this to say Hi!";
using (MailMessage mail = new MailMessage())
{
mail.From = new MailAddress(emailFrom);
mail.To.Add(emailTo);
mail.Subject = subject;
mail.Body = body;
mail.IsBodyHtml = true;
// Can set to false, if you are sending pure text.
mail.Attachments.Add(new Attachment("C:\\SomeFile.txt"));
mail.Attachments.Add(new Attachment("C:\\SomeZip.zip"));
using (SmtpClient smtp = new SmtpClient(smtpAddress, portNumber))
{
smtp.Credentials = new NetworkCredential(emailFrom, password);
smtp.EnableSsl = enableSSL;
smtp.Send(mail);
}
}
Please try this this should work for you
Thank you

sending mail asp.net using google smtp service

before asking i search some possible solution in this site.
I found this Solution that I implemented in my code
But I have an error: when I send a mail with my code (in localhost) I obtain "Invalid Handler" but username and password are correct. Someone can help me?thanks
This is the cdoce
private SendGmail(){
try{
MailMessage mail = new MailMessage();
mail.From = new MailAddress("mymail#gmail.com", "Enquiry");
mail.To.Add(TextBox_mail.Text.ToString());
mail.IsBodyHtml = true;
mail.Subject = "Test";
mail.Body = "Some Text";
mail.Priority = MailPriority.High;
SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587);
//smtp.UseDefaultCredentials = true;
smtp.Credentials = new System.Net.NetworkCredential("mymail#gmail.com", "password");
smtp.EnableSsl = true;
//smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
smtp.Send(mail);
}
catch (Exception errore)
{
elenco_errori.Visible = true;
elenco_errori.InnerText = errore.Message;
}
}

I solved the question
in Application Start, in global.asax I added this code
public void Application_Start() {
System.Security.Cryptography.RSACryptoServiceProvider.UseMachineKeyStore = true;
System.Security.Cryptography.DSACryptoServiceProvider.UseMachineKeyStore = true;
}