Rails Mailer "Net::OpenTimeout: execution expired" Exception on production server only

First, do a direct connection with Telnet:

telnet smtp-relay.sendinblue.com 587
Trying 94.143.17.4...

This is the basic connection troubleshooting, and works with any provider or port. Replace SendBlue and the 587 port with your actual hostname/port.

If you get this error:

telnet: Unable to connect to remote host: Connection timed out

then, the problem isn't in Rails.

In the above example, the problem is in the port number. Services like sendinblue or mandrill (I believe gmail too) don't support the 587 port anymore. "2525" is the new "587".

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If you get a timeout on telnet, check this:

1. hostname: it's usual to people use "smtp.sendinblue.com" instead of "stmp-relay.sendinblue.com", "smtp.mandrill.com" instead of "smtp.mandrillapp.com", and so on.
2. port: 587 is obsolete. Major providers are now using 2525 instead. Major cloud services like DigitalOcean, block outgoing connections to 587 as well. That's why it will work on your pc, but not on your server. I won't even mention the "25" port, which is even more obsolete than 587. Also, some providers use specific non-default ones instead or imap.
3. ipv6 vs ipv4: check if the hostname is being translated as a IPv4. If not, try to disable IPv6 (see others answers).
4. hostname resolution: run the same telnet command on a machine that you know the email sending is working. Check if the translated ip (the xxx part of "Trying xxx...") is the same. If not, go back to your server and replace the hostname with this ip. If works, change your /etc/hosts and force the hostname to use this ip.

I probably had the same issue, my production application didn't send mails although everything in development was working fine. I also got the "Net::OpenTimeout" error.

My problem was that i was using a Google server in production, and it blocks ports 25, 465 and 587 on outbound connections.

Since I was using Mandrill for sending mails, I was able to switch the connecting port from 587 to 2525 and everything is okay now.

Here is also a temporary fix that may come in handy while waiting for your hosting provider to fix the issue:

Add the following lines to /etc/sysctl.conf:

#disable ipv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Now reload the sysctl service using the following command:

sudo sysctl -p

Now the apps are able to send emails again.

You can always know if IPv6 is enabled calling

cat /proc/sys/net/ipv6/conf/all/disable_ipv6

from terminal. Two possible answers: 0 => IPv6 is enabled; 1 => IPv6 disabled.

From: https://serverfault.com/questions/512744/timeout-error-in-all-my-apps-for-every-call-to-smtp-servers