Cerb Email is the mail host beside Hub: real addresses for people who are supposed to have them, not another newsletter pipe.
Hub stays the membership and account layer. When a role includes mailbox access, Cerb asks the mail host to provision an address, then shows IMAP / SMTP / POP settings under Account → Mail so you can use a normal client — and Open webmail for Roundcube at webmail.walkerjacob.com (Hub sign-in; no separate mailbox password in the browser). Message bodies live on the mail host — not in Hub’s content database. Lose the permission or the account, and the mailbox goes away (with a short restore window if staff soft-delete by mistake).
This is separate from transactional mail (invites, login codes, ops alerts). Those still go out through the usual SMTP / Resend path. Mailboxes are optional and role-gated; most friends never see Account → Mail unless I turn that on for them.
Related: WalkerJacob Hub, Cerb CMS, Hub mailboxes, Inside the Hub.