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We changed our micro instance to a small within a VPC and now sending email is sometimes fast and quite a lot of the time very very slow. Its as if our web pages are going to hang and then they eventually complete and sometimes the emails don't appear to arrive.

Also, I had to change the .net code which sends them emails because I was getting an error. I had to start disposing the SmtpClient class. I've never had to do this before! The error went away but the slowness remains.

What might have caused this?

When I do not dispose of the SmtpClient, I get the error...

'Service not available, closing transmission channel. The server response was: Timeout waiting for data from client.'

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Short answer: Use the HTTPS SES API instead of the SMTP endpoint.

Long answer: We used the SES SMTP endpoint in some projects some time ago and found the same issue: When we tried to send large ammounts of emails (and not so large), the SES SMTP endpoint timed out. We tried to solve it deploying some Postfix instances as proxy-relays with no luck. We finally decided to use the HTTPS API and all of our timeout issues vanished. We never had an official answer from the AWS support team, but we think that it is due to some kind of send rate protection in the SMTP software layer implemented in SES.

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Ours was a throttling issue. We had to request AWS to remove it which they did. – Ian Warburton Jun 3 '14 at 15:19

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