Hi @Roy Asterley ,
mailnickname is returned only if included in $select, user - properties.
A Sample request to get mailnickname of users.
GET https://graph.microsoft.com/v1.0/users?$select=id,userPrincipalName,mailnickname
Using the below Graph API we can get user from mailnickname from Azure AD.
GET https://graph.microsoft.com/v1.0/users?$filter=mailnickname eq 'AlexW'
Hope this helps.
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