尝试在 C# 中发送邮件时收到 SMTP 异常

Question

We are receiving an SMTP 4.4.1 connection time out exception every so often when attempting to send mail in C#. So far we have found no pattern as to when the issue occurs, such as the time of day.

The exception thrown is:

System.Net.Mail.SmtpException: Service not available, closing transmission channel. The server response was: 4.4.1 Connection timed out

Up to 30 emails at anyone time are sent and 9 times out of 10 all are sent successfully. When we run into the 4.4.1 exception a number of the emails are sent and the remaining 1 or 2 are not.

Unfortunately, I don't have access to the clients Exchange server, only the application server where our application is running from. So I'm working with the host on this. The Event Log has been checked on the application server, the only thing found was the following warning from a source McLogEvent:

Would be blocked by port blocking rule (rule is in warn-only mode) (Anti-virus Standard Protection: Prevent mass mailing worms from sending mail).

Has anyone came across this issue before or know of a possible cause?

Answer 1

Without any code posted, kind of hard to debug. Have you checked out this similar SO question?

Sending 2000 emails

I found several SO questions with similar errors. My guess is that your server is trying to send too many emails at once and the connection bombs. Try sending a handful at a time; start with one, work your way up to 30 and see if that works. here's my Google search:

https://www.google.com/search?q=The+server+response+was%3A+4.4.1+Connection+timed+out&rlz=1C1CHFX_en__601US628&oq=The+server+response+was%3A+4.4.1+Connection+timed+out&aqs=chrome..69i57j69i64&sourceid=chrome&es_sm=93&ie=UTF-8

Answer 2

I had a similar issue. We would send 1000+ emails in batches of 50 at a time and it would throw the "4.4.1 Connection timed out" SMTP exception every 10 minutes. The issue turned out to be that the SmtpClient wasn't being disposed properly. Apparently, it doesn't send the "QUIT" SMTP command if you reassign it.

Wrong:

SmtpClient client = new SmtpClient();

Right:

using (SmtpClient client = new SmtpClient())