roundcube: enable markasjunk plugin in config.inc.php -> "mark as spam" button appears

train spamassassin: sa-learn --spam /path/to/file

https://www.howtoforge.de/forum/threads/automatisches-spamassassin-training.8881/
https://serverfault.com/questions/614018/suggested-mechanisms-for-user-driven-spam-training

for i in `ls -d1 /var/spool/mailboxes/*/.Spam/cur`; do echo $i; time sa-learn --spam "$i"; done

https://wiki.apache.org/spamassassin/TxRep
https://wiki.apache.org/spamassassin/FrequentlyAskedQuestions