I really was upset when I realized I'd had to use a "SieveBridgeClient" when wanting to use the only sieve plugin existing for TB >=128... No! sieve should be possible *directly* - with or without plugin - I don't care. But if it's forbidden for plugins to communicate directly to external ports it should be integrated into the package. This may be a positive signal to other hosters to enable sieve for customers - maybe not the really big ones... Sieve is important, and would be nice to be able to use it also on mobile devices. Thank you.
If you use IMAP, you cannot really do filtering without using sieve. Local filtering is perfect for POP3, but almost useless for IMAP. I really can't believe that I have to use Roundcube webmail only to edit Sieve filters because TB does not support it (and the plugin is abandoned, and anyway it was far inferior to Roundcube in editing Sieve filters).
Yes, support for ManageSieve (RFC 5804) is essential in today's multi-device environments. Filtering rules and other mailbox organization settings should reside on the server side for IMAP providers that support them. Many users expect a consistent email experience regardless of which device they use to access their email.