I have 2 servers:

• 1st with Postfix, IP <11.11.11.11>, host: mail.1stserver.com
• 2nd one with Phoenix application, IP <22.22.22.22>

The 1st server will reject all emails when a server sending one has no FQDN or perhaps rDNS record -- it's how it's been set up, deliverately.

On the 2nd server I now have the error:

{:temporary_failure, '11.11.11.11', "450 4.7.25 Client host rejected: cannot find your hostname, [22.22.22.22]\r\n"}

First,ly my 2nd server has rDNS which is equal to 22.22.22.22.vultrusercontent.com

Secondly, I've even set the value of hostname manually in the config of Bamboo too:

    config :my_app123, MyApp123.Mailer,
      adapter: Bamboo.SMTPAdapter,
      server: "mail.1stserver.com",
      hostname: "my_app123.com",
      [.........]

What's the matter?

When I send emails via the 1st via a python script on my local laptop or even email client using my this 1st server, all works well.