The Mail sub-tab shows the email thread with this client, pulled from whichever mailbox provider your organisation has connected — Microsoft 365 (Outlook) or Google Workspace (Gmail). Emails sent to or received from the client's address are matched to their record automatically.
From here you can read the existing thread and send new emails to the client without leaving the portal.
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Mailbox connections are set up once for the organisation under Settings → Integrations & Plugins, not per client. See All Plugins for the Microsoft 365 / Google Workspace cards.