When you buy an account described as "USA registered", it was created and first logged in from a US IP. Signing into it from a different country is the single loudest takeover signal Google and Microsoft have — it triggers a verification challenge and often a permanent lock, usually within the first day. Running twenty accounts out of one datacentre IP is the second loudest.

Next Day Mail gives each mailbox a sticky residential session: a stable exit IP per account, distinct between accounts, in the country the account came from, with the mailbox timezone set to match so its sending hours look local too.

Add your provider credentials in Settings, choose the sticky mode and session length, then run the built-in test to confirm the exit IP and that it does not move between calls. 711Proxy has native support: Next Day Mail rewrites the username per mailbox with sessid and sesstime rather than asking you to manage sessions.

socks5://customer-acme-region-us-sessid-<mailbox>-sesstime-30:pw@gate:6000
                        ^ from the country pin   ^ sticky per mailbox

Generic providers work through a {session} and {country} placeholder in the URL, or an automatic username suffix when no placeholder is present.

Set Registered country when you add the mailbox — both the SMTP form and the OAuth panel in Accounts carry it, and it can be changed later on the proxy page. Next Day Mail then rewrites the proxy username to that region for every SMTP and IMAP session, keeps the session token stable per mailbox, and sets the mailbox timezone from the country so pacing windows follow local business hours.

Anything left unpinned falls back to the site-wide default (currently unset — set one so unpinned accounts are not routed at random).

No mailbox is country-pinned yet.

Complete the very first sign-in to a bought account through the same proxy you will pin it to — including the OAuth consent popup and the app-password generation step. Logging in once from your own IP is enough to flag the account, and no amount of correct routing afterwards undoes it.

Do not share one session across accounts, and do not rotate an account between countries "to test". A mailbox that has been challenged is worth re-verifying and resting for a week before it goes anywhere near a campaign.

The doctor in Accounts opens a real connection through the mailbox's own proxy and reports the exit IP, its country, and whether two consecutive calls kept the same address. A mismatch between the reported country and the pin, or a moving IP, means the sticky session is not being applied — fix that before the mailbox spends any warmup capacity.