We are getting substantially more junk mail with the change of our X@RetroCA.com email from Verio/Apollo to RoundCube - what can be done to get back to very low junk mail like we used to have?

Check with Roundcube whether they can filter spam messages on the server. In any case, this isn't a Thunderbird problem.

iPage moved our email from Squirrel Mail to Roundcube, but I can't see a way to block senders. We started with Apollo Squirrel Mail and then Verio bought Apollo and then iPage bought Verio - it's all so confusing to a poor user like me who barely understands any of this stuff.

Bottom line we didn't get any junk mail with Apollo and Verio, but now we are getting dozens every day, and moving each to "junk" doesn't stop the junk from still going into our inbox.

I can't figure out a support email or phone number for Roundcube or iDrive.

I will really appreciate any help I can get to stop the heavy flow of junk mail into our inbox.

Thanks! John Corgiat for Retrouvaille of Nor Cal, a 501(c)(3) non-profit

If Roundcube do not filter spam messages on their server the Thunderbird built-in spam filter is your best bet. https://support.mozilla.org/kb/thunderbird-and-junk-spam-messages