I think most website has certain needs to deliver emails to its users, e.g. account activation emails, private messsage notification, comment notification, etc. Take my site as example, among 5,000 registered users, about 1,500 signed up using gmail.com box, 1,000 using yahoo.com and another 1,000 using hotmail.com. Every now and then I receive complaints from users that they never receive account activation email, sometime it goes to junk folder sometimes it just not show in any folder.
Maybe it's kind of being "throttled" when exceeded maximum number of messages sent from same ip address to gmail.com/yahoo.com/hotmail.com during certain period of time?
I'm using Postfix and there seems no problem with configuration since 90% of emails can be delivered to gmail.com/yahoo.com/hotmail.com boxes successfully.
I noticed twitter is delivering millions of such automatic notifications to its users but I never missed a message from them. How do they archive this? Is there a permanent white list on gmail.com, yahoo.com or hotmail.com?
Thanks in advance.