Conversion to 7BIT required but not supported

[Damian]

> As requested:
> 
> ==== main.cf starts here ====
> ...

>From this config, I cannot think of a reason why postfix would not
announce 8BITMIME. So, if you mail flow is
"postfix:25->amavis:10021->postfix:10026", then I am out of ideas.

There is another possibility that you are chaining amavis instances, of
which the downstream one does not announce 8BITMIME, e.g.
"postfix:25->amavis:10021->amavis:10025->postfix:10026" and

> $interface_policy{'10021'} = 'FIRST';
> $interface_policy{'10025'} = 'SECOND';
> $policy_bank{'FIRST'} = { 
>   forward_method => 'smtp:[127.0.0.1]:10025',
> };
> $policy_bank{'SECOND'} = { 
>   forward_method              => 'smtp:[127.0.0.1]:10026',
>   smtpd_discard_ehlo_keywords => ['8BITMIME'],
> };

or similar.

If that is not the case either, then a more complete log-snippet with
$log_level 5 could help. There you will see a log line that looks like:

> Remote host presents itself as: [127.0.0.1], handles DSN, PIPELINING

Please verify that 8BITMIME is indeed not listed there, when your issue
occurs.