Remote Server returned '550 5.7.705 Service unavailable. Access denied,tenant has exceeded threshold

stefan_hi 

You can refer to the article https://support.microsoft.com/en-sg/help/4498191/tenant-has-exceeded-threshold-error-when-you-send-mail

It describe the causes of it but you can also do the Resolution part.

It says: -> To fix this issue, follow these steps:

• Remove any unknown or unused inbound connectors from Office 365. To do this, follow these steps:
  Go to Exchange Admin Center, and sign in as an Exchange Online administrator.
  Select mail flow in the left-hand frame, and click connectors.
  If any connectors already exist for your organization, you can see them listed here. If you see any connectors that don't belong, you should remove them by selecting the connector and clicking the Trash icon. The following screenshot shows some sample connectors that you should have if you are in a hybrid scenario. All other connectors should be inspected and removed if they are not required.
  An example of connectors to and from Contoso email server

• Check the audit logs to determine whether a connector was created recently. Spammers are known to create connectors, send out lots of mail on that connector, and then delete the connector to cover their tracks.
  Because an admin account was compromised, change the passwords for all your admins.
  To prevent future compromise, enable multi-factor authentication (MFA) for, at least, the admin accounts. For more information, see Set up multi-factor authentication.
  Contact Microsoft Support to unblock the tenant.