The current signature feature in Proton Mail is insufficient for business applications.

In addition to long requested features such as support for multiple signatures, or being able to switch between HTML and Plain signatures on different platforms, other features such as, but not limited to, the following would make Proton Mail a realistic option for larger businesses:

• reply signature option (to avoid spamming a big signature block for every single reply in a thread),
• authenticating signatures (signatures with some kind of validation token; unclear on how to implement),
• a signature catalogue (browse multiple signatures and set for which address they apply as default for regular messages and which ones apply for replies), or
• a signature composing tool (mostly for HTML signatures, would be nice if selectable [and modifiable] blocs were offered which include things like privacy notice language, unsubscribing instructions, holiday notice, etc.; this could be a business account feature exclusive)