Is it possible to set up duplication of a letter from several mailboxes?

Question

Zimbra

Users of my company have to send letters to another company through external mailboxes, since our domain is in the spam list of the contacting company. The contactee company has problems with the spam filter and it takes a long time to delete (I wait 3 weeks).
At the moment, I started creating duplicate mailboxes on gmail and connecting them to the email clients of employees. But ideally, I would like to make a covert sending of a letter from the second box to a specific addressee so that people do not have to make unnecessary movements in the mail client.
Has anyone had a similar experience with the issue?
PS: Email client - Thunderbird, server - Zimbra 8.

Answer 1

www.mcmaster.ca/ctl/tbirdautobcc.htm