The Story alerts are sent from the address:
noreply@dokuga.com
This is really, really dependent on other sites besides Dokuga as well.
Take for instance, if I were to send e-mail to the main e-mail server at one place I work, I can only send a max of, I believe, 20 messages in 10 sec per sending IP address, before the receiving mail server will block connections from that address for 10+ minutes. And how they have the server setup, the time blocked increases each time in a given 24 hour period.
So, it's very easy if there's many e-mails going out on say a story/author alert, to hit such a limit on a remote server. Especially on the free web mail sites. They have a much higher limit than the one I gave, but they do still have a limit from a single address in a given time period; I think some may even have setup limits on "from" addresses too. :/ Completely brilliant since those are trivial to forge, but whatever...
Ahh... spam'ish control type settings. Gotta Love 'Em.
-J