Configure Postfix as backup MX AND local delivery for the same mailbox

I try to explain better...

Actually I have a standalone mailserver (CentOS7+postfix+dovecot) that provide some mailboxes for my domains and I want to add a Backup MX.

As said in lot of doc (http://www.akadia.com/services/postfix_mx_backup.html) a backup MX usually keep mails in his queue and keep try to forward them to the main MX server until it came back online.

But, as I know, is not possible to easy access to these mails in order to read them when the main MX server is down (yes, I can go to /var/spool/mail/somewhere by SSH in case of emergency but this is far away of "mailbox" concept).

So my idea is to deliver a copy of all mails that arrive on the backup MX to a local mailbox too where I can read them with a dedicated webmail. The other copy still be in mail queue ready to be delivered to the main MX when possible (as standard backup MX configuration).

So the question is, how do it? master.cf support customs route for mails (I use a pipe here to forward mails to spamassassin without use amavis, for example) but I wasn't able to locate an how-to/manual/example to do something like that.

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