Can I use email client to move spam email to spam folder?

I am a relatively new Runbox user. Most genuine spam messages go straight to my Spam folder, but very occasionally one appears in my inbox.

A couple of questions. Is moving the message to the spam folder using my IMAP email client enough, or do I need to log into Runbox 7 in a web browser to mark it as spam?

How long does the message need to stay in my spam folder before I can delete it? Am I right in assuming the senders of messages auto-identified and manually-identified as spam are added to a database/sieve filter to improve future identification?

Thank you!