Export solution warnings

Hello all,

In my CRM online 2016 I have created a solution. In this solution i have created a custom entity with a 1-N relationship to Email system entity.  I have created the relationship inside my solution. I see the relationship on both entities, which is correct.

When i try to export the solution i receive a warning message that says :

If i have a relationship between 2 custom entities when i export the solution i don't receive any warnings.

When i import this solution on another environment i don't receive errors.

Can anyone help me with this problem about the warning messages that i receive when i try to export the solution?

Thanks!