Just tested mailgun settings on my end (I use other smtp usually). Here is what works for me (delivery server verification incl test email, then extra test campaigns):
in mwz
hostname: smtp.mailgun.org, protocol: TLS, port: 587
in mailgun
TLS connection: opportunistic
certificate verification: not required
When I tried with their supplied IP, or with certificate verification, or port 465, or SSL: it did not work; but the above does. All campaign inboxed 100%
Here are some other settings used:
(sent two test campaigns for both settings)
with
delivery server settings:
Force FROM: always or never
Reply-To email: as FROM or empty
Force Reply-To: always or never
and
campaign settings:
from (as on delivery server) = reply-to
the resulting header is:
Return-Path: <bounce...
From: from (as on delivery server) = reply-to
Reply-To: from (as on delivery server) = reply-to
So in conclusion, there has to be something either with mailwizz (since the <bounce... ends up as FROM it seems from your error message), or perhaps with the admin@pushpaka email