emails are not delivered, no error message shown

using Rainloop i can receive emails normally, but when i send an email it gets saved in my sent folder, but it doesn’t get delivered to the recipient.
i have tried with multiple email addresses, updated my domain records and configured it to use mailgun, ( when i click on verify host i get: Login successful.), i have also checked if port: 587 is blocked but it is not.

tried following the instructions on the above link, but i get the following error:
failed to add match ‘postfix’: invalid argument

Same here, and above link don’t have anything to resolve the issue.