Backup MX is usually not necessary, especially for home setups. Anything RFC-compliant will retry sending its email to you for days after being rejected.
It's also generally not a wise idea unless you also have your anti-spam/anti-virus setup running on it as well, since things like graylisting can only be done BEFORE the MTA accepts the message.
Spammers WILL know how to search your mx records for things like mailbagging servers and try to exploit them to bypass your security, just don't do it - the sending party will retry.
You can chain SMTP like this: SMTP (backup) -> SMTP (spam filter) -> MTA.
Checking white-lists can take a while and some SMTP servers will give up if they're stuck for over a second, sometimes less (eg when your server checks the spam-lists). Some SMTP servers or DNS server (cought Google caugh) will not try secondary though, so if you can afford it - have just one powerful front server with good uptime and network, or load balance / proxy to internal farm.
My backup MX is set up exactly that way, with an almost identical configuration to my main MX, to avoid this problem. Fortunately the config hasn't changed in over five years.
Running since 1998, haven't been on any blocklists so far.
