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Every mailbox gets a signature the moment it's created, built from two sources: the person's own details, and your domain's company defaults.
This combination is generated once, at mailbox creation time - it's a starting point, not a live template. Editing it afterward, or editing the domain defaults later, won't retroactively change signatures already generated for existing mailboxes.
From Account → Domains, each domain has a "Signature defaults" section for company name, phone, website, and logo URL. Save it before creating mailboxes on that domain so new mailboxes pick it up automatically.
From webmail, go to Settings → Signature to edit the text directly, or turn the signature off entirely for that mailbox.