With `CHUNKING`set as a capability, nginx advertises this capability to
clients at a stage where the SMTP dialog does not seem to be forwarded
to the proxy-target (postfix) yet. Nginx' SMTP parser itself does not
support the `BDAT` command issued as part of a chunke-d dialog. This makes
Nginx respond with a `250 2.0.0 OK` and close the connection, after the
mail-data got sent by the client — without forwarding this to the
proxy-target.
With this, users mail can be lost.
Furthermore, when a user uses a sieve filter to forward mail, dovecot
sometimes chunks the forwarded mail when sending it through `front`.
These forwards then fail.
Removing `CHUNKING` from the capabilities fixes this behavior.
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