The ultra-short answer is: you cannot tell. There’s simply no way to know for sure.
Let’s explore why I say that.
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One reason, or many
The root problem is that there are so many reasons your mail may not get through these days, it’s practically impossible to determine which reason is contributing to your situation.
On top of that, it could be a combination of reasons.
You could, indeed, be blocked
It’s certainly possible that your recipient has blocked you. All that means is that email from your email address is automatically discarded at that recipient’s end.
You will not get a notification; there’s simply no way to tell that this has happened. All you can do is ask the recipient to make sure that they haven’t blocked you. (I assume you’ve been in communication some other way, since you know that your email has not been received.)
It’s possible that your email address has been blocked (or black-listed) by the recipient’s email service, or by a spam filter along the way. Once again, there’s really no way to know whether this has happened. You won’t get a notification – your email will simply be discarded.
Your email could look “spammy”
More often than being blocked, email is often discarded or diverted into a spam folder because it “looks spammy”. Exactly what that means varies quite a bit.
I’ll assume your message isn’t attempting to promote body-part enhancement products or other common spam topics. Even so, it might make sense to review the content of your message, and the terms used, with the eyes of a spam filter in mind.
If your email gets filtered as spam, once again, you’ll get no notification. It’s possible, however, that your message was placed in your recipient’s spam folder, and you should ask if that’s happened. If it has, make sure your recipient marks it as “not spam”, to train her or his spam filter that you’re not a spammer. It may take marking more than one message to do this.
Your email provider could have a bad reputation
Your email service provider’s reputation factors in to all this. If your email host is known to be used to send a lot of spam, then all the email that originates from it suffers.
Once again, there’s no effective way to tell if this is the case. Your email may be silently discarded, or it may be placed in your recipient’s spam folder.
Either way, you won’t hear about it.
Things to try
If you find yourself in this situation, I would start by asking your recipient to “whitelist” you, which may bypass some of the possibilities I’ve mentioned above. Exactly how he or she would do so, and in how many places (email program, spam filter, or email provider) depends on his or her email setup.
Another test would be to send using a different email provider, beginning with simple test messages that are unlikely to be flagged as spam due to their content.
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