[exim] Verify recipient with mailbox quota at ACL time

Hello,

I've an Exim setup with virtual users that are delivered into a maildir
location. Mailbox quota is set up and configured in the appendfile
transport with the quota option.

When an e-mail is received for a user but the mailbox is full, this
results in a bounce message which ultimately gets me on DNS blacklists.
(And it causes backscatter spam which is to be prevented.)

What can I do to not only verify the recipient at ACL time but also the
mailbox quota to detect delivery failure at an early stage and not only
when it's too late and we already accepted responsibility for the message?

I've attached the Exim config in case it's relevant.

This server is running Ubuntu 16.04 but I'll switch to an Ubuntu 20.04
server later this year.

> # exim --version
> Exim version 4.86_2 #1 built 05-Sep-2019 01:49:50
> Copyright (c) University of Cambridge, 1995 - 2015
> (c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007 - 2015
> Berkeley DB: Berkeley DB 5.3.28: (September 9, 2013)
> Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc GnuTLS move_frozen_messages Content_Scanning DKIM Old_Demime DNSSEC PRDR OCSP
> Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmjz dbmnz dnsdb dsearch ldap ldapdn ldapm mysql nis nis0 passwd pgsql sqlite
> Authenticators: cram_md5 cyrus_sasl dovecot plaintext spa tls
> Routers: accept dnslookup ipliteral iplookup manualroute queryprogram redirect
> Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp
> Fixed never_users: 0
> Size of off_t: 8
> Configuration file is /etc/exim4/exim4.conf

Thanks

Yves