Having never seen what the tables for character look like in the database, I can't say for certain what the data structure is like and how much of a data hit each character makes.

But I'd be kind of iffy on this.

As far as I can tell just from playing and limited experience with db's, the only time a player's full character list is hit, is when the account holder log's in, or when mail is sent to a character not currently logged in.

Although, now that I think about it, if each person had a full 10 characters on their account, and there's (for simple math's sake) 1,000 registered accounts, there'd be 10,000 entries for the database to look through if (and I doubt this) they go through entry by entry.

However, the easier way to do this, would simply be to construct the SQL statement to look for a specific String (typically a word or phrase, through it can also be a number), in this case a character name, then simply assign that character entry the value for having mail.

Now, all of this is without having seen any of the the EMU code for the server, so I'm really no expert and KP could probably give a better information on this subject.

Ultimately, it'd cut down on the data stored. I'm not sure though, how much bandwidth it'd free up though.