In a folder based email client, folder actions as you describe them are certainly useful, because they support filing more efficiently.
However, Vivaldi Mail is not a folder based client, and while I do believe that (because people and preferences are different) some functionality supporting folder based workflows should be implemented, it should not get any bells and whistles at this point while we patiently wait for full functionality that supports the core database concept. Which means blazing fast and rock solid search, filtering, and labeling functionality.
Email folders are a huge waste of time, in particular if you do manual sorting. In a nutshell:
If your automated filing system is deterministic / has a clear rule system, then a search using the same rule set will lead to the same results
If your filing system is not fully deterministic or depends on arbitrary choices you made weeks/months/years ago, you didn’t file your email away, instead you have just hidden it somewhere.
If you can’t automate a rule because the email received does not contain very relevant information, consider adding one or more labels (e.g. a project name, or “important”) instead of filing into a folder to support a future search.
