External User Sent a Message to a Skype for Business User, but User was not part of his Contacts?

[Anthony Jarrett]

We're currently using Skype for Business in our Domain, and we recently had a user receive a message from an External user, and started to have a conversation.  Our internal user at first thought it was an internal client, and just so happen to be involved with the information the External client was chatting about.  Luckily, our internal client realized that it wasn't an internal client and promptly Blocked the user.

My question is, how was the external user able to connect with one of our internal clients even though this user was not in his contact list, and never had any interaction with this user?  We did check the internal user's contact list or possibly any other interaction and found none.  I also tested by sending the internal user, a message via an external user account (using my peronsal Skype), while it did add my account to his contact list, my messages didn't reach the internal user.  When the internal user clicked on my external account, it prompted to eithe accept the user or reject.

Any information or insight that you can provide is greatly appreciated.

Thanks,

Anthony

[#2]

Being on a contact list is a control that is only enforced for comms to an external Skype for Consumers service. If the external party is using Skype for Business then it's not required.

As an admin you control who your users can talk to outside your organisation by chosing which federation partners to allow. If this is seen as a challenge then I would suggest you limit federation to companies you trust.

[#3]

Hi Steven,

Thanks for the reply - so if our Federation is not limited, any External party with Skype for Business, will be able to start chat session without being added (of course if external user knows the our internal users e-mail address), correct?

[#4]

That is correct. You can limit access to allow only trusted partners through Skype for business Control panel.
SfB Control Panel -> Federation and External access -> Access Edge configuration -> Uncheck "Enable partner domain discovery" to close open federation.
Then you you will only be able to communicate with defined list of domains under "SIP federated domains" and "SIP federated providers"

[#5]

Got it and thank you for the response!