You still need an index of your mail for searches and extra metadata for tags, threading, unread status etc. So you need a protocol for requesting this search and a format what how the client can use the result of that search to access the matching emails.<p>You need to be able to push email status changes out to connected devices, I don&#x27;t want an alert for an email on phone if I&#x27;ve already read it on my laptop.<p>You need decisions around how attachments are stored. Most of the time you&#x27;ll want to store the attachments separately from the text content of the email so clients don&#x27;t have to download the attachments to access the text, so you&#x27;ll need to decide on how this location would be communicated to a client.<p>Plain files are rather bad for multi-user access, if you have multiple users&#x2F;devices all accessing the same email they&#x27;re going to need some protocol for synchronization so you don&#x27;t get data corruption or lost changes.