PARTIAL SAVE: writes ONLY the 'Mailbox' field of TitlePublicConfigurationModel; all other v1/v2 fields stay untouched. Mailbox - an in-game mailbox that holds VALUE the player claims (not notifications - those are Push). Enabled is the master switch, default OFF. Capacity (default 100) caps visible letters; on overflow the oldest letters WITHOUT unclaimed rewards are removed. RewardLetterDays (30) / PlainLetterDays (14) are letter lifetimes (an unclaimed reward is lost when it expires), ClaimedRetentionDays (7) keeps claimed letters visible. DefaultLanguage (en) is the text fallback. Templates maps a TemplateID to per-language Texts {Subject, Body with {Param} placeholders}, Category (System/Reward/Gift/Social tab), Icon, Banner, Link {FeatureID+Args to open a game screen, or an https Url}, SenderName, ExpirationDays; keys 'system.*' override built-in letters of modules (system.gift, system.transfer, system.transfer_return, system.leaderboard_reward, system.coop_grand_prize, system.coop_object_reward, system.season_unclaimed, system.referral_reward). Gifts.Social maps a GiftID to a friend gift (Grant of items/virtual currencies minted on claim; optional Cost the sender pays per friend, burned; one per friend per period; the period is IntervalSeconds (default 86400) counted from AnchorUtc (default 2026-01-01 00:00 UTC) like fixed-interval cycles of other systems; SendLimit/ReceiveLimit per period, default 20; SenderGate). Gifts.Transfer lets friends send their OWN resources: AllowedResources whitelist (VirtualCurrency or STACKABLE items; MaxAmountPerGift is a CAP - any amount from 1 up to it, entries separately or together), MaxEntriesPerGift (5), IntervalSeconds/AnchorUtc (same period model), SendLimit/ReceiveLimit per period (5), FeePercent (0-90, withheld from what the friend receives, the fee per entry rounds up) and FeeCost (fixed fee on top); both burn only when the friend claims - a declined or expired transfer returns everything charged, MinFriendshipHours (24, against alt-account funnelling), ExpirationDays (7, then returned to the sender). Integrations {Leaderboard, CoopEvent, Season, Referral} (empty = on) bring unclaimed rewards of those modules to the mailbox. Letters and broadcasts themselves are NOT config: they are sent from the dashboard (player letters, segment broadcasts with a SegmentGate and a schedule) or by CloudCode server.SendMail; letters with rewards require the player-admin plan right.