Sadly, this scenario sounds all too familiar! But there’s a good reason:
"When you started your e-mail client this morning, you were prepared for the usual set of correspondence: your daily dose of corporate politics, a dollop of technical emergencies and the background hum of projects under way. Annoyingly, your inbox also contained a few messages advertising products you would never buy, and perhaps a phishing notice warning that your account was frozen at a financial institution where you don’t have an account. Your company has antispam measures in place; surely, the IT staff should be able to keep this junk out of your inbox?
"Perhaps they can, but the task of doing so has become much more difficult in recent years, partly because 85 percent or more of all e-mail traffic today is spam.
[snip]
"The primary directive, for e-mail admins, is ‘lose no mail.’ If that means that an occasional spam message wends its merry way into users’ mailboxes, so be it. E-mail administrators would prefer that users encounter a few annoyances than miss an important business message."
Also, remember that "People are Making Money on Spam," so there may always be strong incentives for spam senders to fight against spam blockers. Who knows, you might want some of that uninvited email….