Login email delayed or not being sent

I use multiple emails to be able to log into my Ghost site with different profiles (paid vs free).

I noticed today that one login email did not get to my inbox for about ten minutes. Another one (sent to a Yahoo address) never arrived at all. I checked the Yahoo address to see if it had been classified as spam, but it apparently never got there at all.

Since I do not have FTP access to the site, is there any way to troubleshoot either issue? And, is Yahoo email not allowed for some reason, so Ghost won’t even send to it?

Update: The Yahoo invite just got here. (20 minutes late)

Question remains: Anything I can do about slow login emails?