problem: if you send a email to a exchange online hosted address you get the answer from the mailserver:
microsoft office 365 error: smtp; 550 5.7.51 TenantInboundAttribution; There is a partner connector configured that matched the message's recipient domain.

solution:
- go to https://admin.exchange.microsoft.com/
-- Mail flow
--- Connectors: check your connecter settings


in german:
- E-Mail-Fluss
-- Connectors: check your connecter settings

additional you can check your antispam settings:
- https://security.microsoft.com/

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