Rotating an exit IP per connection is the fastest way to get a mailbox locked: providers read "same account, new ASN, new country, every hour" as compromise. Next Day Mail therefore derives a stable session token from each mailbox id, so one mailbox always leaves from the same IP while different mailboxes leave from different IPs.

In the 711Proxy dashboard open Residential Proxy → Get Proxy → User & Pass Auth. Create a sub-user, then copy the gateway host, port, username and password. Region targeting is optional — pick a country close to where the mailbox owner claims to be.

Build the URL in this shape (SOCKS5 preferred, HTTP also works). Leave out any sessid yourself — Next Day Mail writes it per mailbox:

socks5://customer-<user>-region-us:<password>@gate.711proxy.com:6000

A healthy result shows the same IP twice for one mailbox, and a different IP when you switch mailbox. If the two calls differ, raise the session length or confirm your plan supports sticky sessions.

Pin each mailbox to the country its account was registered in. A US-registered Gmail that suddenly signs in from Frankfurt reads as a takeover, so the pin keeps every SMTP and IMAP session on an IP from the right country — and sets the mailbox's sending timezone to match. A mailbox can also point at a completely different proxy vendor; overrides always beat the site-wide URL and still get their own sticky session.