You need to ensure is that your IP Address is not on one of the general Block lists which can easily occur in a shared environment. Since you are the provider, you will have to determine which one of your users is causing any given IP address to end up on a Block list .
Similar discussion here https://answers.microsoft.com/en-us/outlook_com/forum/all/block-list-removal-request/d526d35c-1809-47ef-a1cf-bb015dcf1253
Use the delist portal to remove yourself from the blocked senders list and address 5.7.511 Access denied errors https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/use-the-delist-portal-to-remove-yourself-from-the-office-365-blocked-senders-lis?view=o365-worldwide
Hope this resolves your Query !!
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