If you start typing a name, email address or company name and cannot finds someone, then they are not connected to the MX host that you are active in.

Only administrators of the company who pay for MX can add or remove users. If you are looking for access for yourself or a colleague, you will in most cases need to ask your contact from the company to follow an internal process to request an MX account.
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To check who your administrators are, please visit the Account information page.
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