The email postman never rings twice.
Email, Where Art Thou?
Nearly 25% of all commercial opt-in emails never make it to the inbox.
Note that it says "opt-in"... as in people asked to get these emails. Also, ReturnPath has clients that are getting over 90% inbox delivery. That means that somebody is seeing half their email list being delivered directly to spam folders. The crazy thing is that when those companies with low delivery rates keep trying harder by continuing to send blindly to their list, they just make it worse for themselves.
"It is easy to defend yourself from a blind person throwing random punches at you."
Part of the problem is that the large email services have gotten exceptionally good at diverting email you do not want to see. You may think it is about what "spam words" you are using in the email, but spam filtering has gone way past that. When click a "spam" or "junk" button in your email, you are often sending a complaint about the email to your email provider. Email providers pay attention to this. They track who sends every email coming in, and when they get too many complaints (and worse, get them too fast) they start shunting emails from the source to spam folders. The sender never really knows where their emails end up.
Large email services (and even some companies with lots of email addresses) figured out that abandoned email addresses can tell them a great deal about email spam. After some period of dormancy where the real addressee has not looked at the email account for many months, an email operator can assume that anything going into this account has not been requested. This gives email providers a very fast way to figure out what is spam and apply that across their entire population of email accounts. The best way to avoid this kind of filtering is to periodically ask non-engaged list members if they still want your email. If you do not hear back - you must assume they do not.
Non-engaged List Members - if you do not know who these people are in your email lists, you need some help.