doveadm sync backup from old to new server

[Plutocrat]

So specifically in your case:

On 14/05/2020 09.09, Gregory Sloop wrote:
> doveadm backup -D -u mc-user at abc.net ssh root at abc.net -p2200 doveadm dsync-server -u cp-user at abc.net
> 
> mc-user at abc.net is the MC/NEW mailbox/domain
> cp-user at abc.net is the CPanel/OLD user/domain account
> The SSH server of the remote system is running on port 2200.

If you're running this on your source server, I think you have the accounts the wrong way around. Try

doveadm backup -u cp-user at abc.net \
  ssh -p 2200 root at abc.net \
  doveadm dsync-server -u mc-user at abc.net

Or, if you're running it on your target server, maybe you'll have to pull, using the -R switch.

P.