It’s possible for email to not be delivered when spam filters anywhere along the line detect and remove spam server-side. There are two ways this can happen. One way is if the email itself is blocked by a filter. The other way is if an entire organization is filtered for a history of sending emails like in the first example.
The spam you see in Gmail is just a fraction of your actual spam. Most is pre-filtered and you don’t even have the chance to see it. Spam filters have white, black, and gray lists.
- White lists allow every server on it to send them mail.
- Black lists are the opposite, blocking all mail from servers on the list.
- Gray lists are like white, but are built when an email is sent from your server to theirs, meaning, to prevent blocking, send an email to them first.
Good practices to avoid a spam filter are to keep your first email to a person “link and attachment”-free. For example, if you want to send an attachment to someone you’ve never written before, simply ask if you can send them an attachment first and then wait for their reply. Once they reply you are on their gray list.
Having said all that, there has been a DNS attack going on lately that exploits a bug found in BIND. There’s a chance that the email crossed through a network mode affected by the attack and it went to the wrong server.
The most likely cause though is human error where the email address was typed in wrong OR a new device or app was added to check their mail and it wasn’t configured to “leave messages on the server” making them exist only on the device that got their first.
One thing to do is to change their password in case their account has been compromised.
If the organization you are on has been marked as being too spammy, you may have no other option than to switch hosts or email providers. We recommend switching to Google Apps and support Google Apps for most of our clients.