Sendmail as secondary mail server?

Question

Mail server

Is it possible to configure sendmail so that for the specified accounts, mail is forwarded to another mail server that serves the same domain as sendmail, and for all others it is accepted locally as usual.
Or, as an option, is it possible to configure sendmail so that mail for the specified domain is sent through an external server, but incoming mail for this domain is accepted as usual.
The global task is to transfer all accounts to a new server on Zimbra, but since I can’t transfer all the boxes at once, then you need to make the old server secondary (or the new secondary, but the problems remain exactly the same) and gradually migrate to the new server.
From the Zimbra side, everything is clear, there are enough instructions, for example, Split Domain, but how to configure sendmail is not clear. Examining the sendmail documentation and actively searching the net did not bring any positive results, unfortunately.

Answer 1

You can also configure, for example, aliases and forward / copy mail on sendmail to a new mailer.
and slowly drag clients to zimbra, after which you will change and make zimbra through the ip address or the day of the combat MX and put out the old one

Answer 2

Em. Relay option than did not suit?
I won’t tell you with configs, but on one of the works, send soap cleaned spam and then gave all the soap to ekchendzh.
Maybe inside there was toli loc, roofing felts tld, which changed on sendmyl to blabla.com

Answer 3

I did something similar in 2009 just to switch to Zimbra. You just need to be able to write rules in sendmail by hand, you can’t solve it with standard options. As far as I remember, in the set of rules for processing local addresses, sendmail added an arbitrary label to the domain part of the address on the envelope. Then he sent letters with this label to the host with Zimbra. After that, another rule removed the label for local delivery. Unfortunately, I can't tell you more.
Sendmail is generally designed as a universal parser, knowing how to write rules for it, you can implement any logic. True, in our time it is no longer relevant.