A secure email gateway (SEG) is a security service that sits in front of a company's mailboxes and checks every incoming email for spam, phishing and malware before delivering it. Proofpoint, Mimecast and Barracuda are well-known ones, mostly used by larger companies.
Why cold email struggles with them
Gateways are built to stop unexpected email from outside. They scan links, hold unknown senders, and are much stricter than Gmail or Outlook. Cold email to a company behind one often never reaches the person, and the attempt can count against your sender.
How to spot one
A company's MX record usually names its gateway, for example a host ending in pphosted.com (Proofpoint) or mimecast.com. Look one up with the free MX lookup.
What mailzy does
mailzy recognises the common gateways from each lead's MX records and skips leads behind one by default, so your mailboxes spend their daily sends on people who can receive them. You can turn that off per campaign.