after some reports on Em@ilCRX got this message

Warning error 521 yahoo.com closing transmission channel. user is over the limit for messages allowed to be sent in a single day sending Em@ilCRX Abuse complaint <A41>

Many ISP's, including mine, limit how many messages you can send in a single session. Apparently, Yahoo! limits you to so many a day. There is a workaround though it could be a PITA if you get a large amount of sp@m. You simply select a limited number of messages at a time to report. I suppose you could just change their tag to "Received" then change some back to "Spam" when you're ready to send another batch.
A more permanent solution would be to contact your ISP and tell them what you're doing, then ask them if there's a way to allow to send your sp@m reports. Good luck, some ISPs will work with you, others won't.

Seems to work in sections, but could do with selecting more than 1 line at a time to unflag then reflag back again

Amen to that, in triplicate!