Understanding how SmtpClient handles retries - asp.net

Question

I'm working against a GoogleApps account for sending event notifications from my (mvc) web-app. Usually everything works fine. When the system is asked to send more than 75 messages or so I'm seeing replies from the SMTP server:
Service not available, closing
transmission channel. The server
response was: 4.7.0 Try again later,
closing connection. (MAIL)
uf10sm1153163icb.17
However, the system is auto-retrying and anything my system is asked to send eventually (by everything i can tell as this point) making it out. But given how the code generates and sends the emails I don't understand how these re-tries are handled.
I'd like to try to slow down the transmission in hopes that whatever's causing the 'Service Not Available' condition will be placated if the submissions occur asynchronously. But from the looks of the code, it already is since i'm using a Try | catch block.
Here's the relevant bit of my facade code:
foreach (string email in recipientEmails)
{
try
{
EmailGateway.Instance.SendEmail(email, notificationSubject, notificationMessage);
}
catch (Exception ex)
{
Logger.Instance.LogException(ex);
Logger.Instance.LogMessage("ERROR! Unsent email is to: " + email );
}
Logger.Instance.LogMessage("Sent " + notificationSubject + " to " + email);
}
And here's the Gateway code (using System.Net.Mail;):
public virtual void SendEmail(string replyToAddress, string toEmailAddress, string subject, string body)
{
string destinationEmailAddress = toEmailAddress;
string fromEmailAddress = "my#address.com";
bool useSSL = "true";
MailMessage message = new MailMessage(fromEmailAddress, destinationEmailAddress, subject, body);
message.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.EnableSsl = useSSL;
smtp.Send(message);
}
So i'm catching both successes and fails into my logger table. What I don't understand is how I can see a log message for both a fail and then a success condition for the same email address. That indicates a 'retry' and, while i'm not surprised that the SmtpClient (the native .net assembly) can retry without explicit code asking it to, I don't see how my facade's code is being made to log both conditions.

Answer 1

SmtpClient is not retrying to send the email.
In your facade code as it is, you are always logging a success, whether you are getting an exception or not.
You should log success in the try block, otherwise you are catching the exception (logging failure), coming out of the catch block and logging success anyway, which is what you are observing.
foreach (string email in recipientEmails)
{
try
{
EmailGateway.Instance.SendEmail(email, notificationSubject, notificationMessage);
Logger.Instance.LogMessage("Sent " + notificationSubject + " to " + email);
}
catch (Exception ex)
{
Logger.Instance.LogException(ex);
Logger.Instance.LogMessage("ERROR! Unsent email is to: " + email );
}
}