You can't send an IM to external contacts from Outlook Web App
You try to start an instant message (IM) conversation in Outlook Web App with an external contact, also known as a federated contact. You do this from another Lync or Office Communications Server (OCS) organization. However, the message isn't sent, and no error message is generated.
This solution requires the assistance of an Office 365 administrator.
To resolve this issue, create a Contact object in the Exchange Admin Center, and then add the external contact’s IM address to the Proxy Address field. Make sure that the IM address contains the sip: prefix for the IM address of the external contact.
For more information about how to add the IM address as a proxy address to the external contact, use the instructions at the following Microsoft website, but make sure that you use the "sip:firstname.lastname@example.org" format for the address.
To send an IM to the newly created external contact, type the contact’s IM address, and then add sip: to the beginning of the IM address. The message should be sent successfully.
This issue occurs when the IM Address field is populated only with **email@example.com**. Outlook Web App uses the IM address to look for external contacts in Active Directory Domain Services (AD DS) that have a matching Session Initiation Protocol (SIP) address in the ProxyAddresses attribute in AD DS. However, the IM address in the ProxyAddresses attribute always includes the sip: prefix. If the IM Address field is populated only by **firstname.lastname@example.org**, Outlook Web App won't be able to find the user, and the IM will fail.
The IM address must also exist as a SMTP proxy address for the user account. Although the IM address doesn't have to be a primary SMTP address, it must exist so that the user and the SIP address can be located.
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