I fix email as part of my job for my customers.

I have found one of the biggest problems is certain ISP's email systems are buggy, causing email to just disappear at times before reaching its destination.

It can seriously disappear, never reaching its destination, causing much stress and confusion.

When I detect a problem within a certain email company, (usually an ISP Email company's internal blocking filter or programming) I call them to report the problem, but have found more often than not, they are extremely slow to fix it.

Sometimes the easiest way to fix it, is to get a gmail email account. I have never had one lost email, with gmail and it supports many features.