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I have an issue when trying to send emails from my EC2 instance using SMTP to SES. For some reason I am getting sporadic timeout issues, where I can no longer contact the SMTP host. It is important to note that sending through SES works probably 75% of the time.

I will start with some details. My SES account is working most of the time. The sender email has been verified, and my limits have been increased to 10k/day, 5 emails/second. I am under the impression that I would get a limit-specific error if this was related to my limits. For my SMTP configuration, I am using posfix w/TLS. I have posted a very similar post on the official AWS SES forums, but have not had any success there yet. The information for that post is at the bottom of this post.

Here is an example of a failure from this morning. All of the commands I ran below were run from the EC2 instance that I am trying to send mail from. Sorry for the novel-sized post, I just want to make sure I include everything.

An email I sent to myself was not being delivered:

Jun 25 06:16:36 intranet01 postfix/smtp18832: 9E00C230DA: to=<myemailaddress>, relay=none, delay=150, delays=0.02/0.01/150/0, dsn=4.4.1, status=deferred (connect to email-smtp.us-east-1.amazonaws.comhttp://107.22.187.122:25: Connection timed out)

And when I seen the failure I tried connecting to the email host over port 25. I could not connect:

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25  

Trying 174.129.28.151... ^C

A few minutes later my deferred email finally went through:

Jun 25 06:23:14 intranet01 postfix/smtp18861: 9E00C230DA: to=<myemailaddress>, relay=email-smtp.us-east-1.amazonaws.comhttp://184.73.218.23:25, delay=548, delays=548/0.02/0.21/0.36, dsn=2.0.0, status=sent (250 Ok 0000013823cf7441-83710873-e946-4c80-8a54-0dd72bae6f30-000000)

Jun 25 06:23:14 intranet01 postfix/qmgr3972: 9E00C230DA: removed

And now I can connect to port 25:

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25  

Trying 107.20.152.208...  

Connected to email-smtp.us-east-1.amazonaws.com.  

Escape character is '^]'.  

220 email-smtp.amazonaws.com ESMTP SimpleEmailService-222567251  

^]

For kicks, I decided to look at the SES host endpoint. It turns out it is just an ELB cname, with A records that point to interfaces in multiple AZ's.

[email protected] sbin# dig email-smtp.us-east-1.amazonaws.com  

; <<>> DiG 9.7.3-P3-RedHat-9.7.3-8.P3.17.amzn1 <<>> email-smtp.us-east-1.amazonaws.com

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8592

;; flags: qr rd ra; QUERY: 1, ANSWER: 9, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:  

;email-smtp.us-east-1.amazonaws.com. IN A  

;; ANSWER SECTION:  

email-smtp.us-east-1.amazonaws.com. 54 IN CNAME ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com.  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 174.129.200.82  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 184.73.219.75  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 107.20.152.208  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 107.20.160.81  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 107.20.203.50  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 107.22.229.233 

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 174.129.6.189  

ses-smtp-prod-335357831.us-east-1.elb.amazonaws.com. 60 IN A 174.129.28.151

I tried sending another message and it failed. This time I looped through each of the A record addresses that the SES cname returned. I could not connect to any of them. During this time I also tried connecting from my local machine (not my EC2 instance), and it worked just fine.

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.28.151...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.6.189...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.22.229.233...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.203.50...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.160.81...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.152.208...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 184.73.219.75...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.200.82...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.28.151...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.6.189...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.22.229.233...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.203.50...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.179.13...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 107.20.160.81...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 184.73.219.75...

^C

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.200.82...

^C

After waiting around 30 seconds I tried again, this time it worked.

[email protected] sbin# telnet email-smtp.us-east-1.amazonaws.com 25

Trying 174.129.28.151...

Connected to email-smtp.us-east-1.amazonaws.com.

Escape character is '^]'.

220 email-smtp.amazonaws.com ESMTP SimpleEmailService-222567251

^C^[

^]

telnet>

I thought there might be some sort of EC2 SMTP limitation, so I filled out the "sending email from EC2" request form, but it seems silly to me since I am using Amazon's service, as opposed to a 3rd party. So far I have not heard anything back from Amazon after filling out the form.

Does anyone have any ideas? Thanks in advance.

1 Answer

0 votes
by (18.5k points)

EC2 is known to throttle email traffic over port 25 by default. So, it's recommended to use a different port to send email through SMTP to avoid timeouts. In other words, 

EC2 throttling is constrained to port 25. Alternatively, port 587 can be used instead. 

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