Mail System Method List:
This method is executed when the player sends a letter to an NPC.
Return true to suppress the default letter effects (changes in friendship and relationship type), or false to keep those effects.
This method is executed after the player sends a letter to an NPC.
Return true to suppress the default “Mail Sent” message, or false for the default behavior.
This method is used to check if the player should receive any new letter.
Return an array with event names to play when checking the mailbox, or an empty array if there’s no new mail generated by your mod.