I just read this query and don't have time to read all responses, so this might be redundant.
First, I am going to assume you are smart enough to have determined whether or not spam filters are at issue.
Second, I will equally assume that you are familiar with those who have sent you e-mail you haven't received and may well be familiar to those you are trying to send e-mail to.
Third, I will leave out all the terrifying paranoia of malware.
So, then what else can it be?
The answer -- to me -- is the scariest of all. It may be the "personal" choice of your own account provider.
Take AOL. In fact, take them and dump them in the sea. AOL has confirmed to me (and others) that it keeps a special list (a "white" list -- sounds good, doesn't it?) of domains that it blocks en masse unless you yourself have identified a particular sender by listing them as a contact.
Well, these may be dangerous sources, right?
Let's see. What about the Microsoft support entity that mailed out keys to all requesters who wanted free access to MS Media Center when they upgraded to Windows 8? That's right. How do I know? Well, after not receiving a single key at any of my 5 out of seven AOL screennames (MS only allows you one key per address), none of which have any blocks imposed on me, and that checking AOL's own spam bin, and finding nothing from MS, I tried five non-
AOL accounts. They all received keys within days. Still nothing yet in AOL. Then, using the return address on my successful key requests, I add the MS address to my contact list, and immediately received keys on the two remaining AOL screennames I had. I only tried this strategy because I knew about AOL's white list from an encounter with it 5 years ago while ordered software from a vendor it was (wrongfully) suspicious of.
Yesterday, it blocked e-mail from an Apple software engineer providing me support from his personal Apple address. We were talking on the phone steadily and he kept asking me to verify the spelling of my name. Instead I suggested he give me his address, let me add him as a buddy and send him an e-mail, then try again to send me one. Sue enough, following my advice, he got through. But what if we hadn't had the luxury of that phone call?
Well, that happened to me, with one of my utility companies. I figured out the problem eventually, but didn't know the exact address the service personnel would be writing from, so couldn't add a contact (I hadn't formulated by alternate domain strategy yet). I called AOL and they said they wouldn't unblock the sender until the sender made a formal request. The sender's customer service agent said fat chance of that and suggested I change my e-mail provider, whence began my alternate address plan.
So is this White Hat (mostly) secret policy only something AOL does?
I suspect not. I have in the past had persistent failures to communicate using one host or another in which the other hosts handed on my e-mail without delay. I have zero filters set up and allow no e-mail blocking with any of these companies.
Isn't this their right? Not on your life! This secret censorship is yet one more reflection of the largest problem with the entire world-wide web: We still aren't taking it seriously. Somehow, we are still retaining that computers-are-toys attitude with which the entire world of personal computing began.
The old-fashioned attitude that webspace is one big interglobal playground where anyone can do anything he or she wants to is still prevalent.
Well, this playground has become a no-man's zone haunted by predators and malevolent ghouls. They either want to have fun with you in the same way a cat torments a mouse, loading your computer with malware intended to do nothing more than wreck your day. Ha ha, what a comedian! Or they want to steal your money, your identity, your trust, set up liaisons in which you might end up, robbed, raped, or murdered.
You, as an individual, have few rights on this playground. Law enforcement is hamstrung if not actually just plain disinterested because the problem is global and they have too few powers.
We hear excuses that we can't stop conmen who lived in some African nation because their government won't prosecute. Well, d'oh! If they won't prosecute criminals, cut them off from the internet. Watch how long it takes for the government to either cave in or collapse. The internet has become a blood vessel that virtually every semi-organized bureaucracy depends on, totalitarian dictatorship or simple democracy.
But the fix won't happen, because it violates such concepts as free speech and unlimited access. Which is pure tommyrot. Use free speech in a civilized country to con someone out of their life savings and there's a good chance you will lose unlimited access to society. Free speech and unlimited access are successful only they aren't truly what their name implies. That's the result of serious social growth in a maturing society.
Well, the internet is still relatively a two year old child when it comes to a working ethos.
So, if you wondering what's really happening to your e-mail, well ... try switching to a different e-mail address and see what you will see.
In the meantime, you can do the equivalent of twiddling your thumbs by running different virus scans and asking your correspondents to do the same thing. Lots of luck.