We're seeing a huge number of tickets get suspended at the moment.
All of them are from corporate customers and have the standard 'EXT: Warning This Email Is From An External Sender' header.
Speaking to Zendesk Support, it seems I can whitelist individual emails, but it's not possible to do anything to prevent emails with this pretty ubiquitous header from being suspended.
This results in additional work for our support team, as they need to manually vet all suspended tickets at least twice per day, and a bad experience for our customers, who may see their response delayed by up to 24 hours.
There needs to be more granular customisation of these suspension rules.