I have a problem with creating mailaliasses. We have two domains: the primary domain / mailaddress from the user is (example) firstname.lastname@example.org. The second domainname is email@example.com. I have add in the AD in the proxyAddresses the field firstname.lastname@example.org. But, after delta sync from my AAD connector, there is no @corderius.nl in the exchange properties field. What am I doing wrong?
There can be multiple causes for this, a duplicate alias, some other sync error, even the obvious "domain not verified" one. Sometimes when you makes changes directly via ADUC or similar, they are not picked up by the sync engine and you need to "touch" the object. In other words, change another attribute for example set extensionattribute4 and remove it. Then try another sync.
As per @Vasil Michev's suggestions, you should double check that the Custom Domain is registered in the tenant, and of course that you are specifying the proxyAddresses in the correct format.
Although technically the article you referenced should work, it's making some assumptions about other attributes that should be present.
If you are using Exchange Online and syncing your local AD, the only *supported* way (that's to say other solutions like editing using ADSIEDIT can work, but it's not tested or supported by Microsoft) is to use the free Hybrid Exchange server licence to install a copy of Exchange Hybrid as a "management server" for those attributes. Whilst Microsoft is working to eliminate this need, at the moment, that's the best way of ensuring you can manage all those attributes correctly and have all the correct schema updates present to syncronize Hybrid attributes.
"The question of whether a third-party management tool or ADSIEDIT can be used is often asked. The answer is you can use them, but they are not supported. The Exchange Management Console, the Exchange Administration Center (EAC), and the Exchange Management Shell are the only supported tools that are available to manage Exchange recipients and objects. If you decide to use third-party management tools, it would be at your own risk. Third-party management tools often work fine, but Microsoft does not validate these tools."
I would suggest that if you are relying on third-party blogs to work out how to add aliases, you may be better making sure you have things set up the "supported" way. Yes, there are other customers out there who do without happily, but they often have some seasoned Exchange admins or identity specialists ready to provide support in case it all goes wrong.
In the short term, I'd follow those suggestions for a delta sync, and double check proxyAddresses looks similar to:
However there are a variety of other attributes you would expect to see on one the local AD will usually have defined as what Exchange Hybrid calls a "Remote Mailbox".
1. Tenant is registrered, no problem. 2. ProxyAddress is correct: email@example.com 3. We do not have a Exchange here on-premises. See also: https://theucguy.net/office-365-hybrid-license-key/ "If the on-premise deployment is Exchange, you can obtain the hybrid server keys for free by contacting the O365 support team."
Do you have this issue only for one test user or for multiple users? If it's only for one user, you should try with another one.
When syncing users from onpremises, they are synced by AAD Sync to Azure AD and from there there is another sync process that has to happen between Azure AD and Exchange Online. There is a chance that the failure accurs at this stage. You could check in the O365 Admin center (https://portal.office.com/adminportal/home#/users) to see if the respective user has all the addresses there.