We are aware of a known issue with users being unable to invite guests from certain domains via the Guest Access tab in Portal. It seems external domains that include a hyphen are not being synced correctly in the backend, resulting in Portal not allowing guests to be invited via the Guest Access tab.
Latest Updates and Guidance:
This is being investigated under problem record PRB0214273
Please raise a service request with the NHSmail helpdesk (email@example.com), referencing this problem record and the external domain you require guests to be invited from.