Also eigentlich ist meine Original-Abfrage um einiges komplexer, aber für das Beispiel hier hatte ich das Ganze vereinfacht. So sieht die richtige Abfrage aus:
SELECT
u.MailAddress
,e.PortfolioService
,ea.StringValue AS [Serial ID]
FROM
Equipment e
LEFT JOIN EquipmentAttribute ea
ON e.UID = ea.EquipmentUID
INNER JOIN [User] u
ON e.EquipmentUserUID = u.UID
WHERE
u.MailAddress
IN (SELECT
[User].MailAddress
FROM
Equipment
INNER JOIN [User]
ON Equipment.EquipmentUserUID = [User].UID
WHERE
Equipment.PortfolioService LIKE 'M365%'
AND Equipment.Status = 0
GROUP BY
[User].MailAddress
HAVING COUNT(Equipment.PortfolioService) > 1)
AND
e.PortfolioService LIKE 'M365%'
ORDER BY u.MailAddress
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