Whenever there's a mystery regarding a "missing" email, the first thing we look at is if the email is sitting in a spam filter. We always recommend just using one spam filter, preferably "in the cloud" so mail is filtered BEFORE it gets to your mail server, and distributed to your laptop, smartphone, iPad, and wherever else you read your mail.
Technology Advisor Blog
In the first few weeks of using a spam filter, we know it's an inconvenience to have to approve senders when it would be obvious to anyone but a computer that it's mail that you want, but you'll be grateful to have the spam filter when you see all the clutter gone from your mailbox. Note that what appears in your quarantine is only the borderline spam content and that the rest of it is blocked in a "black hole". On average, across all the clients we actively manage, around 80% of all incoming mail is blocked or quarantined as spam!