Recently we had a lucky dip competition in our guild and also ran a guild pool to low level players get the floaters before they was removed from the guild shop.
As such I've sent trades to guild members who were at that time of day off line. Those trades did not time out. After a weeks observation I've concluded that those mails are only timing out at the instant those players open there mail boxes. We had one member who did log into game but did not access his mail and the trade I send him did not time out, it was only a day or so later when he opened his mail box that the mail timed out. As far as I can tell this has only been a problem since the end of the football event as trades sent out to donate footballs to lower level players behaved as normal.