I never touched the free sea coin that I recieve from mail because I dont know that to exchange it for and I dont do sea content but I planned in the future, so I never touched that free coin on my mail. Today I checked my mail and now its all gone. after I recieve 500 cron stone and guardian pre creation event rewards.

Greetings!

If I recall correctly, that mail has an expiration and it will be deleted if not collected. If I'm wrong, how about try sending a ticket? https://sea.pearlabyss.com/Support/Start

Hello~

Yes, like Florentino said - it does got expiration. You could try sending a ticket, maybe they will help you with it.

In the future, make just to claim anything in the in-game mail before the next updates even though you didn't use it in the nearest time. We didn't know it'll stay or will be gone/remove after the regular maintenance time - as some of it, doesn't tell you when the mail will be expired. Just to be safe!