Work Email Spam Reduction

I’m drowning in junk that keeps slipping into the work mailboxes on my own domain, so I need an email-security specialist to tighten things up fast. The accounts are not on Gmail or Outlook; they’re hosted directly on my domain, and right now the basic filtering isn’t doing the job... (Budget: $10 - $30 USD, Jobs: DNS, Linux, Network Security, System Admin, Technical Support, Web Security)