Mail client cannot connect to SMTP server

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1

Nextcloud version: 27.1.3 (Snap)
Operating system and version: Ubuntu 22.04
Apache or nginx version: Don’t know
PHP version: Don’t know

I have just set up a new Nextcloud instance using the snap package and everything but the SMTP mailer works flawlessly. When I try to test my settings using the “send email” button, I always get the response

"A problem occurred while sending the email. Please revise your settings. (Error: Email could not be sent. Check your mail server log)"

… after some delay of ~10 seconds (timeout?).

Unfortunately I do not have access to the logs of the remote SMTP server but I verified that it is configured correctly by using the same SMTP credentials on other devices and testing services.

The SMTP server supports unencrypted/STARTTLS on ports 25/587 as well as SSL on port 465, but every configuration in Nextcloud yields the same result.

I have checked all the log files in /var/snap/nextcloud/current/logs, but pressing the “send email” button does not produce output in any of those files, despite 'mail_smptdebug' => true, (which is why I did not include log output here).

I suspect some kind of connectivity issues, which is why I tried telnetting into the SMTP server from the Nextcloud instance. For some reason, telnet seems to prefer an IPv6 connection which is not working (I don’t know why). After around a minute, the connection times out and telnet tries the servers IPv4 adress, which successfully connects.

root@cloud:~# telnet my.mail.server 25
Trying *IPv6 adress*...

... WAITS ~1 MIN UNTIL TIMEOUT ...

Trying *IPv4 adress*...
Connected to my.mail.server.
Escape character is '^]'.
220 mail.server.alias ESMTP *provider information* Thu, 30 Nov 2023 18:40:55 +0100
EHLO my.cloud.server
250-mail.server.alias Hello cloud.server.alias [*cloud server ip*]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP

My hypothesis is that the Nextcloud instance is having the same issues when trying to connect to the SMTP server through IPv6, but I don’t know how to confirm this due to the lack of logging output.

Is there some way to force the Nextcloud SMTP client to use IPv4? Or can I reconfigure my system so that it prefers IPv4 connections over IPv6 in general?

Thanks in advance for any help!

2

Try

but notice the changes to restore them later, if it does not help.

1 Like
3

That did the trick!

(I permanently disabled IPv6 by reconfiguring grub)

I tried adding

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.tun0.disable_ipv6 = 1

in /etc/sysctl.conf before, but that didn’t help.

Thanks a lot!

4

Maybe your network (router) cannot forward IPv6-requests to internet. So requests to the mail server run into “nothing”.

5

Sounds plausible.

But then why would the router lease IPv6 adresses to my devices…?
Topic for another day/thread :smiley:

Thanks again for the help :slight_smile:

6

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