Hello. I sent a notification about your question to the company employees, expect an answer from them here in the comments.

In the meantime, you can set up sending a copy of the letter to the addresses you need from the mailbox to which the letters arrive. For example, you need to send letters to mail A, B and C. But the program allows you to send letters only to A. Then, in the settings of mail A, create a rule for forwarding a copy of the letter to B and C.