From time to time an ISP will refuse to accept e-mails from a particular source due to excessive numbers arriving from that source, possibly due to spam activity. This happened to me some years ago when I e-mailed my brother-in-law in Germany (he never answers the telephone)! to advise him of the death of his aunt. I got the failed to deliver message five days later and he missed his aunt's funeral. It happened again more recently but over a trivial matter and the failed to deliver message arrived the next day.