In AppMail, each install uses unique header prefixes which are generated at install time.
Seems that latest version of SpamAssassin does not like this, so if you run your emails through SpamAssassin, for example on mail-tester.com, you might get something like:

As you see, this new rule takes almost 3 points from our score.

In order to fix this, we need to use the traditional X- header prefix only, so no more X-Whatever- prefixes.
Luckily, with AppMail this is simple, and you don't need to wait for an application update to have this fixed.
Open the file apps/common/config/main-custom.php which is basically the file generated at install time which contains your database connection info.

Inside your file, you will see something like:

return array(

    // application components
    'components' => array(
        'db' => array(
            'connectionString'  => '...',
            'username'          => '...',
            'password'          => '...',
            'tablePrefix'       => '...',
        ),
    ),
    // params
    'params' => array(
        'email.custom.header.prefix' => 'X-Abcd-'
    ),
);

We need to adjust the X-Abcd- part, so simply edit the file, and make sure you remove the Abcd- part, so that your new prefix is just X-, like:

return array(

    // application components
    'components' => array(
        'db' => array(
            'connectionString'  => '...',
            'username'          => '...',
            'password'          => '...',
            'tablePrefix'       => '...',
        ),
    ),
    // params
    'params' => array(
        'email.custom.header.prefix' => 'X-'
    ),
);

That's it, save the file and re-run the mail-tester test, this time the results will look much better:

NOTE:
Because AppMail uses these headers to process bounces, please make sure you change the header prefix only after at least 3 days after you have sent your last campaign, basically give the app at least 3 days to process any incoming bounces using the old header prefix.