How getmail deals with delivery errors and with bounce e-mails

Hi all:

First of all, many thanks for getmail.

I am a mail server newbie, and I am trying to set up a multidrop feed to Dovecot.

Despite all trouble and all the investigation I had to do, I actually got it working. 8-)

getmail is just launching /usr/lib/dovecot/deliver (actually a symlink to dovecot-lda) to deliver the fetched e-mails.

But I have not managed to generate e-mail bounces yet, when the envelope recipient does not actually exist in Dovecot.

Dovecot can allegedly generate a bounce e-mail itself with option 'submission_host', but apparently, it cannot supply SMTP credentials when doing 
that. I would rather not install an unauthenticated relaying SMTP server inside the LAN. Even if I knew how to configure Postfix or anything similar, 
and it were listening on localhost only, it would still be an open SMTP relay for anybody with a local user account on that machine. If I understand 
it correctly, you cannot set access permissions on TCP listening sockets.

I could try to use Dovecot's option sendmail_path instead, but it is not even documented how Dovecot uses such a tool.

Before I dig more on the Dovecot side, I wanted to know that getmail's take on this kind of delivery problem is.

I mean, if getmail fetches an e-mail, and dovecot-lda errors by writing to stderr and exiting with a non-zero status code, then getmail knows that 
delivering that particular e-mail has not worked.

getmail however does not seem able to generate a bounce e-mail itself. Or have I missed something?

I hope that getmail will then move to the next e-mail. It is not actually documented. I will ask later another question about this kind of behaviour, 
because I have seen a failing e-mail in a different scenario stopping the processing of all other e-mails altogether.

Will getmail just try the failed e-mail the next time around? Forever? Or will it give up after dovecot-lda has failed a few times?

What happens if getmail is invoked with --idle=INBOX ? Will it never retry that failed e-mail? Or will it, only once every 30 minutes or so? That 
timeout is what RFC 2177 recommends for IDLE.

I could not find any documentation either about how long --idle will wait before timing out and doing another idle iteration. Or does getmail quit 
after those 30 minutes?

Thanks in advance,
   rdiez