POP3 Account and SpamAssassin Training Steps

I access my Runbox accounts via an email program on my Mac. While I use SpamSieve locally, I would like to train the Runbox spam filter to reduce the spam that reaches my computer.

Recently my spam has increased either due to election season or an “unsubscribe” link I used because I thought that the sender was bona fide (most likely both reasons LOL). So I have been logging-in to Runbox webmail and tagging spam in my Inbox, with the idea that SpamAssassin would intercept future email from those spammers.

It isn’t clear to me that my plan is working, From what I have read about the Runbox spam system, I am wondering whether it is only possible to train the spam filters when using IMAP.

Any advice/links regarding how I should proceed will be appreciated.