We are trying to find the best solution for this. Unfortunately it depends on the mail provider and they all have some issues. For example, the one we were using last week Sendgrid (currently testing a new provider) was having issues delivering emails to Hotmail and live.com accounts.
The current one, seems to delay emails to Yahoo...
I Get Email Reminders of Proposals but Not When I Initially Get Them
Re: I Get Email Reminders of Proposals but Not When I Initially Get Them
For over a year I have not received any notifications from watzatsong. And now, today, I have more than 500 that have just landed in my spam folder. I would like to point out that I always check my spam folder and that they were not there before.