Hi @Alex Kyslov ,
The IMAP configs (Get-ImapSettings -server ... | format-list) of the old CAS 2013 and the new CAS 2019 are identical.
According to the explanation in this blog (It's talking about the client connectivity in the Exchange 2016 coexistence with Exchange 2010, but based on my knowledge, this part about POP and IMAP can be applied to Exchange 2019 coexistence with Exchange 2013 as well.), in coexistence scenario, IMAP and POP clients would be proxied from the newer version of Exchange Client Access component to a target server.
Since in your case, the target mailbox is the old CAS 2013, you'll need to ensure that the InternalConnectionSettings
maps to the target server's FQDN.
So, I'd recommend trying to configure the InternalConnectionSettings
value as aforemention using the Set-ImapSettings cmdlet and then check the result.
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