Zygo checks to see whether messages sent (whether from the web or from your phone) have been received by the people they have been sent to - if they haven’t, we show the ‘Unread by’ flag next to the message. We do the check by requesting ‘delivery receipts’ from the receiver’s phone; these get sent back to us as soon as the phone has received the message. Immediately after you have sent a message from the web, we show the message you sent on the messages page, but at that point it’s unlikely we will have received any delivery receipts. The message list refreshes every 30 seconds, by which time the receipts will have come back to us and the ‘Unread by’ flag will disappear for most people - unless, of course, they have their phone switched off and we still haven’t received any delivery receipts, in which case they really will be unread.