There's nothing in ISO27001 that would specifically preclude what you describe.
At a high level provided you have conducted a risk assessment applicable to the use of the email proxying service and the 'output' is consistent with your risk acceptance criteria it will not be incompatible with ISO27001.
Thinking more about specific control areas, as a starting point there are relevant controls/advice in 'Supplier Relationships', there are likely others that will have relevance as well (refer to the standard and decide based on the proposed solution).
Also, since you appear to be within the EU, you may want to ensure you have covered data protection requirements in the contract with the supplier (Microsoft should have standard contract clauses that are compliant with EU data protection requirements).
If you do not already have a copy I would recommend getting hold of ISO27002 which provides more detailed explanation about the controls in ISO27001.