<![CDATA[Apps using requiresValidCertificate in the sendmail addon cannot send emails on a domain configured to use an external SMTP relay]]>I ran into an issue while packaging a Django app that performs strict certificate validation on the hostname when sending emails, installed on a Cloudron instance that uses an external SMTP relay (another Cloudron) for outgoing mail.

When requiresValidCertificate is set to true in the app manifest, Cloudron replaces the value of CLOUDRON_MAIL_SMTP_SERVER with the FQDN of the mail server, as described in the docs. The problem is that when the domain's mailing settings are configured to relay through an external SMTP server, Cloudron doesn't seem expose any mailing ports, so the app ends up trying to connect to a host that simply doesn't accept SMTP connections.

Steps to reproduce:

  1. Configure a domain on Cloudron to relay outgoing mail through an external SMTP server
  2. Install ntfy using that domain
  3. Log in to the admin account and add an email address to it
  4. The verification email will never be sent

ntfy doesn't appear to log an error in this case, the connection attempt to the SMTP server most likely just times out silently in the background.

]]>https://forum.cloudron.io/topic/15841/apps-using-requiresvalidcertificate-in-the-sendmail-addon-cannot-send-emails-on-a-domain-configured-to-use-an-external-smtp-relayRSS for NodeFri, 04 Sep 2026 17:54:42 GMTWed, 19 Aug 2026 16:59:43 GMT60<![CDATA[Reply to Apps using requiresValidCertificate in the sendmail addon cannot send emails on a domain configured to use an external SMTP relay on Sat, 22 Aug 2026 20:04:17 GMT]]>Thanks for the report. I have fixed this with 1cd0140808ccaadc665e9f06b3910a8abbbdc167

]]>https://forum.cloudron.io/post/128427https://forum.cloudron.io/post/128427Sat, 22 Aug 2026 20:04:17 GMT<![CDATA[Reply to Apps using requiresValidCertificate in the sendmail addon cannot send emails on a domain configured to use an external SMTP relay on Sat, 22 Aug 2026 08:54:53 GMT]]>mm, something else must be off. The code path for sending email with or without relay is the same from app's point of view.

]]>https://forum.cloudron.io/post/128411https://forum.cloudron.io/post/128411Sat, 22 Aug 2026 08:54:53 GMT<![CDATA[Reply to Apps using requiresValidCertificate in the sendmail addon cannot send emails on a domain configured to use an external SMTP relay on Wed, 19 Aug 2026 17:07:29 GMT]]>Hello @milian.hackradt
That is a tricky one.
Since when using requiresValidCertificate: true the CLOUDRON_MAIL_SMTP_SERVER variable points to the domain itself e.g.: mail.cloudron.dev.
But when mail.cloudron.dev is not hosting the mail server but is using a relay, there is no smtp/imap port to connect to.
So when using the other variables from https://docs.cloudron.io/packaging/addons#sendmail for automatic configuration, the app itself tries to auth against something that is not there.

Interesting edge case you found there!

]]>https://forum.cloudron.io/post/128303https://forum.cloudron.io/post/128303Wed, 19 Aug 2026 17:07:29 GMT