Clicking 'Set as Reply' for Remote mailbox leads to targetAddress attribute value change in the MMC console.
Product defect TF00440910.
A script based workaround is available. The script policy will act as a guard and remove unwanted targetAddress attribute modification.
Enclosed you will find the workaround script module text, it ignores targetAddress changes for the objects having the type of Remote Mailbox (msExchRecipientTypeDetails= 2147483648). Please refer to the article below for more details:
To apply the proposed script based workaround:
1) Rename the enclosed file (remove .rename);
2) Import the script as PowerShell PolicyScript to ARS;
3) Create a script policy and link it to the affected node.
This will be fixed on a future release of the product.