can it be that 4xx errors are not being reported back to the client?

[None]

When I reject messages with a 5xx code and error text, the sender is
most of the time being notified and given the error text.
It looks like the 4xx is not being relayed back to the sender (probably
because the server is instructed to try again later)

Is there an error response that informs the sender while the server
still keeps trying?

[Claus Aßmann]

None wrote:

> It looks like the 4xx is not being relayed back to the sender (probably
> because the server is instructed to try again later)

Correct.

> Is there an error response that informs the sender while the server
> still keeps trying?

Depends on the MTA of the sender - sendmail uses
Timeout.queuewarn
for this - see the docs for details.

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[None]

>
>> Is there an error response that informs the sender while the server
>> still keeps trying?
>
> Depends on the MTA of the sender - sendmail uses
> Timeout.queuewarn
> for this - see the docs for details.
>

Are RFC's defining things like what codes should generate response back
to the sender?

[Claus Aßmann]

None wrote:

> Are RFC's defining things like what codes should generate response back
> to the sender?

An MTA should send a "DSN" if a mail couldn't be delivered.
AFAICT nothing else is defined in the RFCs, but maybe someone
else read them more carefully -- or you can check RFC 2821/5321(?)
yourself.

Whether other notifications are sent is outside the scope of
the RFCs because it is "local policy/configuration".