I ran into a business acquaintance recently. He seemed to be annoyed with me. When questioned further, he said he had sent me an email and why had I not replied to it? Well, since I had not received any, I let him know that. He showed me the sent mail.
Some brainstorming later, I clicked on the 'Spam' folder in my mail account. Lo and behold, the mail from my friend sat there prettily, amongst all those others exhorting me to try this or buy that.
Now, I correspond with this person quite often, and no, his email id is not 'blocked' or otherwise relegated to spam.
Moral of the story - do peep into your spam folder once in a while, else you might lose out on friends and business in the long run!