Try websieve if your backend is cyrus-imapd. It would be nice if there was a way (maybe there is) to provide a sieve interface in roundcube, though websieve works pretty well, users for teh most part seem to "get it".
> Is there an autoresponder for rc?
An autoresponder, for .. a mail client? That'd only be able to respond
when you read mail and never any other time?
I suspect there is a better tool for the job, ask your system admin.
If you are that admin, and it is unix, then look at "vacation".
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