Sometimes corporate spam filters and email firewalls don't like seeing a big group of the exact same emails being sent to multiple recipients inside a company. It looks like spam. Or maybe it's used to seeing you sending one or two personal emails to a client in the company, but when it sees an email campaign come from the Benchmark server purporting to be "from:" you, it thinks it is coming from an impostor sender.

Whatever the case, sometimes corporate email firewalls can be really strict. They'll block or "blacklist" an email server for all kinds of reasons.

Sometimes the block is temporary, for instance, if the email server feels too many emails are coming from one server too quickly. Sometimes the block is permanent, and the only way to get around this is to ask the IT group in charge to add Benchmark Email to their “whitelist”.