Amazon SES remote error: tls: user canceled, 421 Connection closed by server. Too many messages in one session

Question

I've been using SES to send 2-3k emails per day without issue. When I increased it to 50k+ I started getting a bunch of these errors.

```
421 Connection closed by server. Too many messages in one session
remote error: tls: user canceled
```
Two different errors, but they both consistently occur at about the same rate.

My SES limits are well above the settings I'm using. My limit is 500k/24hrs, 120/s. I'm sending at 100/s. I tried lowering the send rate to 50/s and I still get the errors.

I thought it might be the recipient email server (gmail, hotmail, etc.) rejecting my emails, but looking at the subscribers it failed for, they're very diverse. IE: they're not all from gmail or hotmail, but many providers.

I found a discussion https://stackoverflow.com/questions/49111453/sending-email-with-aws-ses-via-smtp-error-421 which says it might be solved by changing the port from 465 to 587. I'm considering trying that, but many sources, including Amazon https://docs.aws.amazon.com/ses/latest/dg/smtp-connect.html, say that 465 is more secure.

EDIT: I tried STARTTLS 587 but got the same errors.

Accepted Answer

Hi THere

Can you provide some details on how your application is sending mail?  Is it using some sort of keep alive to hold a connection open to the SMTP server?  If so, you might want to try limiting that.  Also, do you have any retry/backoff in your application code?

See https://docs.aws.amazon.com/ses/latest/dg/troubleshoot-smtp.html#troubleshoot-smtp-response-codes

> You should retry SMTP requests that receive 400 errors. We recommend that you implement a system that retries requests with progressively longer wait times (for example, wait 5 seconds before retrying, then wait 10 seconds, and then wait 30 seconds). If the third retry doesn't succeed, wait 20 minutes, and then repeat the process. To see an example of an implementation that uses an exponential retry policy, see How to handle a "Throttling - Maximum sending rate exceeded" error on the AWS Messaging and Targeting Blog.

Amazon SES remote error: tls: user canceled, 421 Connection closed by server. Too many messages in one session

Relevant content