Backup MX servers do this every day. One of the commonuses of differing MX priorities is to provide a backup MX server to accept mail on the behalf of your mail server if it is overloaded or unavailable. This has been used in the past to provide reliable mail transport to mail servers that are not always connected.
One downside has traditionally been that backup MX servers were generally much less stringent in their connection level spam filtering/blocking (since the downstream server is generally responsible for that, and they may be a backup server for multiple downstream servers), so it became common for spammers to send directly to lower priority mail servers to take advantage of this and bypass a lot of that active filtering at the eventual destination. Expect a lot of spam to queue up.
In your case, you actually would control the backup MX and the eventual destination (if it indeed is a separate SMTP server), so that's less of a problem. You could just put a pretty harsh timer on the queued mail, and throw it away after 24 or 48 hours. Then again, you could probably do all this almost identically by replacing the SMTP server run on the client device with an IMAP client, and just have delivery end at your server.
One downside has traditionally been that backup MX servers were generally much less stringent in their connection level spam filtering/blocking (since the downstream server is generally responsible for that, and they may be a backup server for multiple downstream servers), so it became common for spammers to send directly to lower priority mail servers to take advantage of this and bypass a lot of that active filtering at the eventual destination. Expect a lot of spam to queue up.
In your case, you actually would control the backup MX and the eventual destination (if it indeed is a separate SMTP server), so that's less of a problem. You could just put a pretty harsh timer on the queued mail, and throw it away after 24 or 48 hours. Then again, you could probably do all this almost identically by replacing the SMTP server run on the client device with an IMAP client, and just have delivery end at your server.