Hi Emilio

For this purpose we used dovecot proxy & mysql with a view that would prepend the pass scheme depending on if an account is already migrated (pointing to old or new IP): create or replace view v_proxy as concat(if(proxyhost='<ip of new server>','{MD5}','{PLAIN}'),<old or new password>) as password, [... other fields you might need...] from accounts;

Basically you ignore the default_pass_scheme and let mysql prepend the correct scheme based on whatever property you like, in your case the domain name. You could do this in the target database directly but if you need the different pass schemes only while migrating (and update the old passwords to the new format) I'd recommend using dovecot proxy so you don't have to change your target user table. This also has the advantage that users don't need to set new hostnames in their clients - you simply point the hostname to your dovecot proxy and after the migration is completed point it to the new system.

hth Oli