Customized activities were introduced in CRM 2011. Customized activities are customized entities that allow you to select the option “Define as an activity entity” when you create the entity. Customized activities are displayed and behave like other CRM out-of-the-box activities (Email, Task, and others)
Mailbox entity was introduced in CRM2013. Unlike CRM2011, in CRM 2013 the Mailbox entity is now linked to activities in 1:N Relationships.
At times of transition from one CRM version to another, the ISV will maintain 2 development environments one on CRM 2011 server and the other one on CRM 2013, especially if you wish to provide CRM 2013 users with the new CRM 2013 UX, while still supporting customers using CRM 2011. This works well for new customers. Existing customers that installed the ISV solution on CRM2011 and then upgraded to CRM2013, when they import exact same solution for CRM 2013, the solution fails with the following error message.
The entity relationship role of the referencing entity is required when creating a new one-to-many entity relationship tst_test_mailbox_sendermailboxid.
Mind you, we only refer to ISV solutions that include customized activities.
We found out that the CRM managed solution exported from a server that is an upgrade from CRM2011 is different from the same solution exported from CRM2013 which is not an upgrade of CRM 2011 server. Whether it’s a bug or a feature, the details on how to replicate this phenomenon
In the meantime, you can overcome the issue by deleting the relationship link in the xml, as described below. I wish to remind readers that the changes made to the CRM solution using methods described in this post are unsupported by Microsoft and could cause problems during updates and upgrades to Microsoft Dynamics CRM.
Unzip the solution
Using XML Notepad open the file: customizations.xml
Click Edit>Find and look for: mailbox_sendermailboxid
Delete this Entityrelationship node as per this drawing
by DNN Integration Dynamics CRM