How to send an email from Outlook to two Exchange organizations at once?

Question

System administration

Given: One user, two mailboxes in different Exchange organizations, one profile. When this user tries to send a letter to recipients in different Exchange organizations, he receives an NDR, since he uses the Outlook cache, where legacyExchangeDN boxes are stored. Items in the cache are valid! You cannot disable the cache, the user of the "boss nervous" class. Whether it is possible to reconfigure Outlook for him so that SMTP addresses are stored in the cache instead of legacy? It all the same at attempt to send on SMTP at first substitutes legacy if finds.

Answer 1

Probably to you, as someone familiar with your infrastructure, it seems that the task is described ideally. But for me, for example, as an outside observer, after several readings, the puzzle did not work out.
Firstly, there is no point in disabling the cache, if the recipient is internal, then Exchange uses the address in the legacyExchangeDN for delivery.
Secondly, it uses x500 addresses only for internal recipients, and to external organizations (whether they are at least Exchange or Sendmail, they are delivered via SMTP), and now about the two Exchange organizations, what did you want to say, I would like to know more. And where is the box of the user himself (in the third?)? How are these two organizations related in general?