I have rewrited this one of the oldest code (legacy of wowd-mangos-trinity, has been written more then 15 years ago).

Early, when player doing "log in world", server should to do SQL-query with subqueries of all his mails and mail items. Each *** time. 


Mail Init and Mail Items containers



and ofc delete operations with subqueries also 
 

Just try to imagine : what to do with server, if player will collect mails near 20-30k and will spam log in/out ? 

unfortunately, so you can crash all other projects

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examples of new functions :

 
example from AuctionHouseMgr