Gone are the days when people had a simple email address from their ISP and everything just worked. Today your email address is a virtual and aggressive port into your systems from where continual efforts are made to compromise you with viruses, malware, trojans and phishing schemes or bombard you with irrelevant and unsolicited advertising. You have no options but to continue with the email communication system and defend yourself as best you can. For the sake of brevity let's just refer to this collectively as "spam".
Your ISP (and those of your correspondents) as well as several other parties have been forced to initiate systems that identify spam and either delete it (in some cases) or redirect it to spam buckets, filtered folders and other temporary containers. This is supposed to reduce the flood of spam that you receive, but inevitably this also results in false-positives where genuine business mail is misidentified as spam and generally lost. This can have considerable cost implications for business and it happens all the time.
This works in both directions. The mail that a user sends to a correspondent may never reach its destination. Being identified as spam usually means that the sender (which address is usually forged) is not sent a bounce message and and the user is therefore unaware that the recipient has not received it. Most often it would sit undiscovered in the recipient's upstream spambucket (at the ISP and not in the recipient's mail client) and would be automatically deleted after a specified time. A similar process happens with incoming mail and the user would be similarly unaware of the diverted mail.
It takes an understanding of these issues and constant monitoring and modification to ensure that these losses are minimised. We follow a policy of analysing the site and domain first to get a measurement of mail deliverability which is a technical process. We then assess the general user competency in their understanding of best practices in terms of ensuring they receive their incoming mail.
It is then possible to plan a course of action to correct these issues and minimise the mail losses.
These are standard services extended to our hosted clients.