Some companies pass incoming email through a security service such as Proofpoint, Mimecast, Cisco or Barracuda. These are very strict with cold email: emails to them rarely arrive and still use up your daily sending.
What mailzy does
mailzy recognises these services from each company's public mail records and shows them in the Leads page's Security gateway column and filter. By default, a campaign doesn't email anyone behind one, neither the first email nor a follow-up. Each such lead reads Skipped, with the service named.
A lead whose company hasn't been checked yet (usually a few minutes after it's added) waits as "Waiting for a security check", then goes out or is skipped.
Emailing them anyway
Switch on Send to leads behind a security gateway in the campaign's Settings, under Protection, and pick any gateways to keep skipping. Changing it brings back the leads it skipped; if the campaign has already finished, press Resume. Launch tells you how many leads won't be emailed.
Some services work inside the mailbox and leave no public trace, so "None" doesn't promise there's no filter.