composer.json doesn’t update with the latest versions of Mautic. If you run composer update, there are missing dependencies for the SAML bundle.
Solved by grabbing from github, but then there are version errors. Solved by adjusting the versions.
When I send mail:
(www-data is Apache user, I send from PHP Mail) - this did not happen until 2.6.1
Not a massive issue, as I have catchall for my bounce folder set up, so I will still get them, but it would be nice if this gets fixed in future updates.