Are there any current, free high availability strategies for Dovecot ?
I would say the major issue with high availability is having your data readily available and updated across your nodes. To solve this I would go full object storage with this ceph integration for dovecot. Ceph is really high availability storage solution, no doubt. The plugin consist of storing your email as objects and communicates directly with ceph via rados. So your mail server nodes are around 8GB while the TB's are stored in ceph storage.
The only thing you need to do is use this dovecot director or some other solution to bind users to a specific mail node. However, next to the storing of emails in object storage you have the indexes, currently this is solved by storing these on cephfs. I had some issues with cephfs that is why I am rethinking what to do. Maybe I will put indexes on rbd images and sync those somehow between nodes. I also noticed some write amplification on my setup, I can't remember exactly but I think in my case I had twice the storage being used, I need to retest this though. So with the default 3x replication in ceph, I think I came near 6x times...