There's lots of Twitter users that have migrated here recently, and a good chunk of them have discovered the Mastodon Twitter Crossposter, which means your timeline is probably littered with sloppy looking retweets.
And you can rid yourself of them with this one simple filter:
Follow
Okay, some notes since I didn't test this exhaustively before I posted about it:
The filter above works pretty well, but I found it to be a bit more effective to fitler "RT" without the "@", and the also filter "twitter.com", which will also filter any link to Twitter.
But if you're here to get away from Twitter, that should really do the trick.
