Buenas
Lo primero de nada que debo decir es que este tipo de preguntas debes hacerla en la sección de programación .NET, no en programación general.
El mensaje de error es 'self-explanatory' (se explica por si solo), te está indicando que el servidor requiere autentificación SSL así que debes activarlo, o también es posible que hayas escrito mal algún otro parámetro como las credenciales.
si no muestras tu código es imposible ayudarte en la corrección del mismo.
de todas formas te muestro un ejemplo por si estás haciendo algo mal:
' GMail Sender
' By Elektro
'
' Usage Examples :
' GMailSender("Username@Gmail.com", "Password", "Email Subject", "Message Body", "Receiver@Address.com")
'
''' <summary>
''' Sends an e-mail through GMail service.
''' </summary>
''' <param name="Username">Indicates the GMail account username.</param>
''' <param name="Password">Indicates the GMail account password.</param>
''' <param name="Subject">Indicates e-mail subject.</param>
''' <param name="Body">Indicates e-mail body.</param>
''' <param name="Addresses">Indicates the address(es) to send.</param>
Private Sub GMailSender(ByVal Username As String,
ByVal Password As String,
ByVal Subject As String,
ByVal Body As String,
ByVal Addresses As String)
Using MailSetup As New System.Net.Mail.MailMessage
MailSetup.Subject = Subject
MailSetup.To.Add(Addresses)
MailSetup.From = New System.Net.Mail.MailAddress(Username)
MailSetup.Body = Body
Using SMTP As New System.Net.Mail.SmtpClient("smtp.gmail.com")
SMTP.Port = 587
SMTP.EnableSsl = True
SMTP.Credentials = New Net.NetworkCredential(Username, Password)
SMTP.Send(MailSetup)
End Using ' SMTP
End Using ' MailSetup
End Sub
Traducción al vuelo a C#:
// GMail Sender
// By Elektro
//
// Usage Examples :
// GMailSender("Username@Gmail.com", "Password", "Email Subject", "Message Body", "Receiver@Address.com")
//
/// <summary>
/// Sends an e-mail through GMail service.
/// </summary>
/// <param name="Username">Indicates the GMail account username.</param>
/// <param name="Password">Indicates the GMail account password.</param>
/// <param name="Subject">Indicates e-mail subject.</param>
/// <param name="Body">Indicates e-mail body.</param>
/// <param name="Addresses">Indicates the address(es) to send.</param>
private void GMailSender(string Username, string Password, string Subject, string Body, string Addresses)
{
using (System.Net.Mail.MailMessage MailSetup
= new System.Net.Mail.MailMessage()) {
MailSetup.Subject = Subject;
MailSetup.To.Add(Addresses);
MailSetup
.From = new System.Net.Mail.MailAddress(Username
); MailSetup.Body = Body;
using (System.Net.Mail.SmtpClient SMTP
= new System.Net.Mail.SmtpClient("smtp.gmail.com")) { SMTP.Port = 587;
SMTP.EnableSsl = true;
SMTP
.Credentials = new Net
.NetworkCredential(Username, Password
); SMTP.Send(MailSetup);
}
// SMTP
}
// MailSetup
}
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saludos