Queued Emails Fail in AWS VPC Environment

Question

I have a system that sends out emails and queues them if volume increases.

If it's 1 or 2 emails per hour, it sends out fine.

On busier times, the queue gets backed up and soon email sending starts to fail.

I've load-tested the SMTP server from my local - no problems there.

However, when I try to do the load-test from my EC2 server
- which is in a private subnet of a VPC
- where outgoing traffic goes through a NAT
- and where we've set up our own DNS server

the first few go out fine - and the rest starts to throw errors.

Here's the C# exception ToString()

    System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebExcept
ion: Unable to connect to the remote server ---> System.Net.Sockets.SocketExcept
ion: A connection attempt failed because the connected party did not properly re
spond after a period of time, or established connection failed because connected
host has failed to respond [SMTPServerIP]:25
at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddre
ss socketAddress)
at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Sock
et s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state,
IAsyncResult asyncResult, Exception& exception)
--- End of inner exception stack trace ---
at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object ow
ner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket
6)
at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Gener
alAsyncDelegate asyncCallback)
at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate
asyncCallback)
at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncD
elegate asyncCallback, Int32 creationTimeout)
at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint)
at System.Net.Mail.SmtpClient.GetConnection()
at System.Net.Mail.SmtpClient.Send(MailMessage message)
--- End of inner exception stack trace ---
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at ConsoleApplication2.Program.Main(String[] args) in c:\ConsoleApplication2\
Program.cs:line 49

I'm pretty sure it's not a code issue - most likely a setting on the NAS or DNS. Can someone point me in the right direction?

Answer 1

After some more googling - it seems to be a throttle on AWS' end

Source: http://aws.amazon.com/ec2/faqs/

Q: Are there any limitations in sending email from EC2 instances?

Yes. In order to maintain the quality of EC2 addresses for sending email, we enforce default limits on the amount of email that can be sent from EC2 accounts. If you wish to send larger amounts of email from EC2, you can apply to have these limits removed from your account by filling out this form .

Better details here:

Amazon EC2/SES SMTP Timeout