There are lots of things in Second Life that send email, but as many of you have discovered, that email doesn't always get to the intended recipient. There are a lot of reasons for email from SL not being as reliable as we'd all like, and the Lab is working on fixing as many of those as we can, but some of them need your help.
One of the most important reasons mail doesn't get through is that many Second Life accounts have bad email addresses - when something in Second Life sends to one of those bad addresses, it generates errors. The number of those errors causes many network providers and spam detectors to block even perfectly good email if it comes from Second Life. To fix that, we need to cut down on the bad addresses, so… over the next few months we will be converting all the sources of email in Second Life to not even try to send to an address that has not been verified.
To check whether or not your address is verified, go to your Account -> Change Email page; look next to your current address there. You should see a green checkmark and tag: √ Verified. If you do, you're all set and none of our changes will affect you negatively. If you don't, there will be a button you should use to send yourself a verification email; when you get the email, click on the link in the message to verify your address. Changing your email address will automatically send a verification email.
If your address is verified, you should not have any problems with our changes to sending behavior - hopefully, you'll begin to see more reliable messages.