for (MailDB myDB : maillist){
    if(myDB.getReceiver().equals(player.getName())) {
        mmo.sendMessage(player, "&aFrom: &c%1$s &a- &c%2$s", myDB.getSender(), myDB.getMessage());
        myDB.setRead(true);
        maillist.remove(myDB);
    }
}

Error:

20:29:50 [SEVERE] java.util.ConcurrentModificationException
20:29:50 [SEVERE]       at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
20:29:50 [SEVERE]       at java.util.ArrayList$Itr.next(Unknown Source)