Anyone who has ever done laundry knows that there is a parallel universe that can be accessed through your washer and occasionally, socks will make it through this wormhole that the agitator creates. The same thing happens to email, only the wormhole to the other world is located somewhere in the spam folder.
Spam filters are meant to make life easier and to filter out all the junk mail that has taken over what was originally a very useful form of communication. Because spammers are constantly innovating to find ways around spam filters, the programmers of the filters are constantly making them more difficult to get through. The result of this is that inevitably, some of the good emails get caught up in the filter and chucked out before they are ever received. If the email that gets filtered out is something like Aunt Sally's bean soup recipe, it's not that big of a deal. If it's an email from a hiring manager who is responding to your resume submittal, that could be catastrophic.
Here at our firm, we send a LOT of emails so we see the hazards of the spam filters on a regular basis. We make it a practice to check our spam boxes daily before emptying them to make sure that nothing important was weeded out. Job seekers need to make a habit of doing the same. An important email from a hiring manager or a recruiter, if deleted by the spam filter, is an opportunity lost. If the sender receives no reply from the job seeker, he/she will simply go on to the next candidate.
Another email problem that we see and which puzzles us every time is when a job seeker will change his/her email address in the middle of the job search. A similar problem is when a job seeker uses several different email addresses during the job search. It's is always best to start and finish your job search with one main email address. In fact, if you can have one email address that you maintain only for career purposes, you will enjoy the best response. If an email bounces or disappears into the parallel universe, a recruiter won't go to the trouble of trying to run you down - there are just too many good candidates in the market to waste the time. You have to be available on the first shot. Make sure you monitor your email communications closely to be sure it is as efficient and easy as possible.