Gmail and Outlook both ship a genuinely good set of triage tools, and most teams underuse them. Filters and rules route on sender, domain, subject and body text. Labels, folders and shared inboxes give a sales mailbox real structure. Priority Inbox and Focused Inbox learn from what you open. If you have never sat down for an afternoon and written proper rules for sales@, do that before you evaluate any software, including this one.
Here is the wall you hit. Every one of those tools acts on a pattern you defined in advance. A rule is a statement about senders you already know: this domain, that phrase, this address. It executes perfectly and it executes only against what you anticipated.
A company that has never written to you before matches no pattern. It has an unfamiliar domain, no history in your mailbox, and a subject line you did not predict — which is an exact description of a new customer, and also an exact description of a fraudulent purchase order. Both arrive as unmatched mail. Both land in the same undifferentiated pile as the newsletters. The one category of message you most need pulled out is the one category rules structurally cannot pull out, because the defining feature of a new buyer is that you have nothing on file about them.
That is the gap. Not "mailbox rules are bad" — they are good at what they do. The gap is that the highest value message in the mailbox is the one with the least prior information attached to it, and rules run on prior information.