HELP - cannot add email to the exchange with outlook classic 2021 and 2024

That message means Outlook cannot resolve the account to a mailbox recipient in the Exchange address book; manual server entry usually will not fix it. First confirm the user can sign in to Outlook on the web and that an active mailbox, not only an identity, exists for the exact address. In Exchange, verify the recipient’s primary SMTP address and that HiddenFromAddressListsEnabled is false, then allow changes to replicate. Test Autodiscover for the affected domain with Microsoft’s Remote Connectivity Analyzer and correct any DNS, certificate, or endpoint failure it reports. On the PC, install current Office updates, then hold Shift while starting Outlook, create a completely new profile, and add the account by email address so Autodiscover supplies the settings. Compare with a known-working mailbox on the same PC. If every mailbox fails, investigate Autodiscover or client licensing; if only one fails, focus on that recipient object and mailbox provisioning.