Oooooh. It works now! You showed me the right path 
I uninstalled mutt: apt remove mutt
Installed back mailx: apt install bsd-mailx
And then used your sorcerry:
chown axigen:axigen /opt/axigen/bin/sendmail
chmod 6755 /opt/axigen/bin/sendmail
Not sure why, but it works.
I did 755 permissions, because users outside of axigen group needs to send messages too.