[pcmill]

If I understand it correctly it never let's the "enduser" connect directly to the email accounts. It always has a service in between. That way as far as Gmail or Outlook knows it only gets requests from one or a handful of IP addresses.

[clbrmbr]

That’s the point: a small number of IPs accessing many accounts could become suspicious. (But may not be trivial to distinguish from legitimate usage due to NAT.)