Discourse version at localhost: NOT FOUND

Discourse with Proxy nginx. Site worked including email, but emails are no longer sent without log error in discourse. Trying to trace the problem down, I found

Discourse 2.7.0.beta1                                                                
Discourse version at irusweg.de: Discourse 2.7.0.beta1                               
Discourse version at localhost: NOT FOUND                                            
==================== DNS PROBLEM ====================                                
This server reports NOT FOUND, but xxx reports Discourse 2.7.0.beta1 .        
This suggests that you have a DNS problem or that an intermediate proxy is to blame. 
If you are using Cloudflare, or a CDN, it may be improperly configured.              

I am aware that this is not a Discourse problem, but rather one of proxy configuration. Nevertheless, any hint would be helpful.

For a robust test, get an address from http://www.mail-tester.com/                                
Or just send a test message to yourself.                                                          
Email address for mail test? ('n' to skip) [admin@xxx.de]: d@mymail.de       
Sending mail to 
Testing sending to xxxxx using myserver:587.                        
= ERROR ========================================           
                                    UNEXPECTED ERROR                                              
500 5.5.1 Invalid command                                                                         

If your site works you can ignore the proxy error.

The other error is also unhelpful, as the tool is useful only for the most common problems like a login issue.

Are there logs at the mail server? Is it receiving mail?

Has something changed since when the mail was working?

Thanks for your comment on the proxy error. However:

  • I can sendmail from the host without problems
  • After installing a ephemeral sendmail in app, trying this inside the app container gives the following:
echo "Subject: sendmail test" | sendmail -v mymail@mymail.de
... Connecting to [] via relay...
... Deferred: Connection refused by []

Any hints where too look further would be appreciated.