I keep getting the mail notification sound despite not having any unread mail. It comes about every thirty minutes. I have cleared out by inbox, deleting even read mail, and I'm not a member of any fellowships or FCs. Obviously it's not a game-breaker, but it is kind of annoying.

I saw someone else had a similar issue a while back (https://forum.square-enix.com/ffxiv/threads/360589) but they didn't say how they resolved it. It started around the beginning of Endwalker for me and has continued pretty regularly. The sound comes in dungeons, cut scenes, housing, pretty much anywhere in the game. I will add that I haven't gotten any FC invites in ages, which just may be because I'm not a sprout anymore but I suppose it might have something to do with it.