It's very random-seeming. If the server that is trying to deliver to yahoo happens to be on their "greylist", those emails can be delayed for hours or days. Yahoo can't/won't do anything about this. Most people's ISP have multiple outbound, as an example: if your isp has a.smtp.xxx.com through f.smtp.xxx.com, and d.smtp.isp.com is on the sh*tlist, outbound mails from xxx.com to yahoo have a 1/5 chance of hitting this. very very irritating...