Webmail and email-hosting portals — corporate webmail, ISP mail, school mail — are below, with 0 guides. Each "official site" link points to the mail provider's domain, checked manually.
If you cannot reach your webmail, the most common reason is that the organisation has moved to a different platform (for example, a school migrating from a self-hosted system to Microsoft 365 or Google Workspace). The IT helpdesk is the right contact — not the previous webmail's "forgot password" link, which may no longer be live.