Further each Microsoft Exchange-Online tenant has a unique tenant-ID in form of a UUID ([RFC 4122](https://tools.ietf.org/html/rfc4122)). **ExOTA-Milter** extracts the tenant-ID from the *X-MS-Exchange-CrossTenant-Id* email header and uses it as a *mandatory* authentication factor.
Further each Microsoft Exchange-Online tenant has a unique tenant-ID in form of a UUID ([RFC 4122](https://tools.ietf.org/html/rfc4122)). **ExOTA-Milter** extracts the tenant-ID from the *X-MS-Exchange-CrossTenant-Id* email header and uses it as a *mandatory* authentication factor. Since September 2022 Microsoft did not set this header anymore reliably.
```
[...]
X-MS-Exchange-CrossTenant-Id: <UUID-of-tenant>
@ -125,17 +125,18 @@ Finally it´s the combination of all of the above discussed aspects which may re
* matching for client certificate´s CN (ExOTA-Milter)
* verification of DKIM signatures providing *Authentication-Results* header (another milter, e.g. OpenDKIM)
* consideration of DKIM verification results per sender domain (ExOTA-Milter)
* matching for tenant-id provided in *X-MS-Exchange-CrossTenant-Id* header (ExOTA-Milter)
* *OPTIONAL*matching for tenant-id provided in *X-MS-Exchange-CrossTenant-Id* header (ExOTA-Milter)
For small setups, with not so many domains, the JSON-file policy backend (default) may be sufficient. If you´re an email service provider (ESP) maintaining a lot of customer domains in a LDAP server, you may want to use the LDAP backend instead. Details regarding the LDAP backend can be found [in the LDAP readme](LDAP/README.md).
# How about a docker/OCI image?
## Using prebuilt images from [dockerhub](https://hub.docker.com/)
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