sympl-mail: /srv/example.com/mailboxes is required to accept mail
Due to the changes in Exim in Debian 11, the config now expects the /srv/example.com/mailboxes directory to exist for incoming mail, and fails if it doesn't (ie: theres aliases or default forward, etc).
Reported in https://forum.sympl.host/t/mail-aliases-in-config-aliases/234
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