Here's a recent suggestion that was published into a KB article!
Sophos XG Firewall: How to export and view emails that are in the Mail Spool in MTA mode
[locked by: SupportFlo at 12:56 PM (GMT -7) on 2 Oct 2018]
[unlocked by: SupportFlo at 2:03 PM (GMT -7) on 16 Oct 2018]
[locked by: SupportFlo at 2:03 PM (GMT -7) on 16 Oct 2018]